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APPLICATION OF CONJUGATED POLYMERS TO MULTI-ELECTRODE ELECTROCHROMIC DEVICES

By

ECE UNUR

A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT

OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

2008

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© 2008 Ece Unur

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To my father, Irfan; my mother, Havva; my sister, Necibe; and my brother, Lutfu

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ACKNOWLEDGEMENTS

Graduate school has been one of the most difficult times in my life. In addition to leaving

home thousands miles away, I had to survive all the obstacles that come in to my way. I could

have never achieved this without my family and friends’ support. Thus, I would like take a

moment to acknowledge all those who have touched my life along these five years.

First and foremost, I would like to thank my mother; Havva Unur and my father; Irfan

Unur for dedicating themselves to raise three good kids. I respect them in working very hard,

instead of enjoying their youth, just to offer us a better education, better life and better future. I

sincerely thank them for being caring, loving, and supportive to all around them and standing the

best sample of what should an honorable person be. I would like to extend those thanks to my

brother; Lutfu Unur (Minik Gus) and my sister; Necibe Unur (Neco Can) for their support and

encouragement over the years. They were always just a call away. Even at those times, I was

ready to cry, hearing their cheerful voices took me back to those peaceful breezy endless summer

nights in Mudanya. I always wished, I were there with them, enjoying Minik Gus’s jokes. Neco

Can, thanks for teaching me that sometimes what may seem like an obstacle might be a plus for

your life if you are wise enough to look at the issue from a different point of view. Believe me it

helped a lot. Again, thanks to you all for caring for me more than anyone else could in this

world. It might be a difficult sentence to repeat at all times, but I would like to take this chance

to say it out loud, “Sizleri canimdan cok seviyorum.” Without you, I would never have made it

here today.

I would like to truly thank my advisor, Dr. John R Reynolds for his patience,

understanding and utmost respect for my work. I also thank him for all the opportunities he

provided, such as attending conferences all over the USA and making me part of CIBA project,

which helped me find my niche in science. I will always remember the great trip to his land

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California where Stefan, Aubrey and I had the chance to see the real ‘end of the tunnel’. I would

like to thank Dianne Reynolds for being such an understanding wife, thus Dr.Reynolds could pay

attention to all our academic and personal needs. I also want to thank her for being so friendly

and caring since the day I started in this group. Sometimes a little is bigger. All her kind words

during the first lonely years made me feel welcomed.

I would like to thank my supervisory committee members Dr. Kenneth B. Wagener, Dr.

Paul H. Holloway, Dr. Randy Duran and Dr. Valeria Kleiman for their interests in serving on my

committee. I would like to thank Valeria for her guidance and valuable discussions. I would like

to thank Dr. Wagener for trying to call my family when I burst into tears at my first year in his

office. Such kindness could never be forgotten.

I would like to thank Roger J. Mortimer for his collaboration, giving me feedbacks through

the establishment of high quality work and taking his time to proofread my work. I thank him

for his encouragement and trust. Friends like him make life easier, fun and they make the lab a

bearable place.

I want to thank my colleagues at Ciba Specialty Chemicals, Joe Babiarz, Mike Craig,

Jennifer Jankauskas, Nancy Cliff, Shujun Wang, I-Chyang Lin, and David Yale for not only the

incredible work we accomplished, funds and supplies, but also for their support and for

welcoming me at all my visits. They have given me the opportunity a graduate student could

rarely have; they gave me the chance to take part in the real professional life.

I would also like to thank Dr. Ryan M. Walczak and Dr. June-Ho Jung for taking their

precious times to have coffee breaks with me. They made me believe in myself again and have

me started in the CIBA project with a great enthusiasm.

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I would like also to acknowledge all the people I worked with, and who helped enrich the

work presented in this dissertation: Dr. Ben Reeves, Dr. June Ho Jung, Ciba Specialty

Chemicals, Dr. Stefan Ellinger and Pierre M. Beaujuge for sharing their precious babies

(polymers and monomers) with me, Dr. Jeremiah Mawaura, Dr. Nisha Ananthakrishnan, Dr.

Christophe Grenier and Dr. Avni A. Argun for training me, Dr. Evrim Atas, for being a great

support through all my studies and giving me insight, Dr. Amelia Dempere, for sharing her

experience and wisdom and caring enough to recognize your pain among all the others.

I would like to thank all the members of Reynolds’ Group. You have all made it a great

experience. I also would like thank the members of George and Josephine Butler Polymer Labs,

for the help and making sure everything runs properly.

I also would like to thank to the administrative staff, Cheryl Googins; for showing me the

warmth that I could see from my family, Sara Klossner; for helping me at all the ‘last moment’

runs with a patience that could only be seen in a prophet, Gena Borrero,for dealing with all the

weirdoes to get our orders in, Tasha Simmons and Lorraine Williams, who had been there with

me at some parts of the journey.

A special thank you goes to my labmates for their help and support, and my coffee break

companions, Maria, Svetlana and Nate. I also would like to thank Dr. Svetlana Vasilyeva for the

valuable discussions, collaboration, for sharing her wisdom, and for being such a dependable and

honest friend at the most difficult times. I wish to thank Laura Moody for always cheering me

up. I would like to thank Ken Graham for his collaboration and always smiling face.

I cannot omit my MS. advisor and mentor, Dr. Levent K. Toppare. Without his trust and

guidance, I would have not been here, doing PhD today. He turned my life into an outstanding

experience. I would like to thank him for helping me to learn who I really am.

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I would like to give my special thanks to the TURKISH MAFIA. Without the members of

GOBEKVILLE (Nihan, Mete, Memet, Akin, Eray, Gogce, Sibel, Arpat, Stefan, Meryem,

Huseyin, Emel), I couldn’t have enjoyed but endured this journey. Gogce and Stefan, you have

been my warmest place to run in this lonely town, you are my family. You all made me feel

close to home. Nihan, you are a very special friend. Words are not enough to tell how lucky I

feel to be your friend/sister. I also would like to thank my dear Anaklara for being a great friend,

neighbor, support and teaching us everything necessary to survive in USA, from partying to car

shopping. I would like to thank you for giving me a basket before the sunrise and sending me to

collect candies to experience an American tradition, red carpet nights and so many others. Last,

I would like to thank Perihan Balikci Brown, for listening to me patiently, being there for me at

all the good and bad times, and for sharing her life with me. I would like to thank my dear

Georgios Pyrgiotakis (Chef) for all the great shopping and cooking experience, for showing me

all the good parts of this town and for all the movie nights. I also would like to thank DJ Webby

for the great music he shared with me. I want to thank my dear girl friends Debra Anderson and

Rania Habib, you were the ones who held my hand first, pulled me out of the darkness and

started my beautiful life in Gainesville. I will remember you; Debra, Rania and Chef for this all

through my life. I also want to thank the greatest dude ever, Dr. Jorge Chaves Benavides and his

perfect wife Dr. Sara Lane and their little princess Victoria for bringing a joy into our lives and

their friendship. I also would like thanks all the girls, Laurel, Sara, Delmy, Fedra, Ozge, Marie

and Gokce for our special event ‘Girls’ Nights’. It was always fun to be with you and know that

there was someone to depend on. I also would like to thank the Fashion Police Daniel Kuroda.

Thanks to him, I have a pair of red shoes and a red purse, now. Delmy, you and your daughters,

your sweetness and friendship have influenced in so many ways. I will never forget the chats at

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the Reitz Union. We know that a cup of coffee and a piece of cookie can build great bounds. I

would like to thank all the ‘Pacozs’, Atay and Ceylan, for their endless support and cheering me

up even if I am not an ‘outstanding person’. I also would like to thank to the ones who kept in

touch and provided their support from thousands of miles away, Serdar T. Demir (I haven’t

exchanged that many emails with anyone else in my life), Ipek Kerman, Pelin Edinc, Pinar

Yilmaz. I would like extend my thanks to the members of ‘yawshax’, Basak, Serra, Yeliz, Neco,

Elif and Hande. Even looking at our pictures and reading emails made me laugh my head off,

you have the power to turn the most stressful times into the most fun ones. Hande, you have

never let me feel alone, you called even at your most hectic times to make sure that I was OK, I

really appreciate everything and all the trips you have done to meet me.

Last, but not the least, I would like to thank Dr. Mete Yilmaz for everything he has done

for me without expecting any rewards. He has been my best friend, greatest companion and

support. He showed me, together, you can survive anything and most importantly helped me

find the true friendship. I am proud and honored to be part of his life. He made me believe that,

for the first in my life, someone other than your family could love you unconditionally, just for

whom you really are.

In the end, our close friends get us through the difficult times. They all have been my best

friends for the last five years and they made me feel closer to home. I feel so lucky to meet them

and I love them so profoundly.

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TABLE OF CONTENTS page

ACKNOWLEDGEMENTS ................................................................................................................. 4

LIST OF TABLES.............................................................................................................................. 12

LIST OF FIGURES ............................................................................................................................ 13

LIST OF ABBREVIATIONS ............................................................................................................ 17

ABSTRACT ........................................................................................................................................ 18

CHAPTER

1 ELECTROCHROMISM AND COLOR IN CONJUGATED POLYMERS .......................... 20

1.1 Introduction ........................................................................................................................... 20 1.2 Electrochromism ................................................................................................................... 21 1.3 Color Control in Conjugated Polymers ............................................................................... 23 1.4 Electrochromic Devices ........................................................................................................ 27 1.5 Color and Colorimetry .......................................................................................................... 30 1.6 Color Mixing Theory ............................................................................................................ 38 1.7 Structure of Dissertation ....................................................................................................... 40

2 EXPERIMENTAL TECHNIQUES ........................................................................................... 41

2.1 Chemicals, Materials and Instrumentation .......................................................................... 41 2.2 Electrochemistry ................................................................................................................... 43

2.2.1 Electrochemical Setup................................................................................................ 43 2.2.2 Electrochromic Polymer Film Formation ................................................................. 45

2.2.2.1 Electrochemical deposition ............................................................................. 45 2.2.2.2 Spray or drop casting ....................................................................................... 45

2.3 Electrochromic Film Characterizations ............................................................................... 46 2.3.1 Spectroelectrochemistry ............................................................................................. 46

2.3.1.1 Dual method ..................................................................................................... 47 2.3.1.2 Electrochromic Devices .................................................................................. 47

2.3.2 Colorimetry ................................................................................................................. 47 2.3.3 Composite Coloration Efficiency (Tandem Chronocoulometry

/Chronoabsorptometry) and Switching Times........................................................... 48 2.3.4 Optical Stability of Polymer Films and Devices ...................................................... 49

2.4 Standard Two-Probe Surface Resistivity Measurement ..................................................... 49 2.5 Dual Film Technique ............................................................................................................ 50 2.6 Electrochromic ECD Construction ...................................................................................... 51

2.6.1 Window Type Absorption/Transmission Electrochromic Devices (ECDs) ........... 51 2.6.2 Pseudo-Three-Electrode ECDs .................................................................................. 52 2.6.3 Three-Electrode ECDs ............................................................................................... 52

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2.6.4 RGB Color Space Five-Electrode ECDs .................................................................. 53

3 DUAL-POLYMER ELECTROCHROMIC FILM CHARACTERIZATION USING BIPOTENTIOSTATIC CONTROL .......................................................................................... 55

3.1 Color Mixing ......................................................................................................................... 56 3.2 Choosing A System for Dual-Polymer Technique by Fundamental Properties

(EDOT, ProDOP and ProDOT-Hx2) ................................................................................... 57 3.2.1 Film Deposition .......................................................................................................... 58 3.2.2 Polymer CV and Scan Rate Dependence .................................................................. 60 3.2.3 Spectroelectrochemistry ............................................................................................. 61 3.2.4 Tandem Chronocoulometry and Chronoabsorptometry .......................................... 64 3.2.5 Colorimetry ................................................................................................................. 66

3.3 PProDOP/PEDOT and PProDOP/PProDOT-Hx2 Dual Systems ...................................... 69 3.4 Conclusions ........................................................................................................................... 73

4 APPLICATION OF BIPOTENTIOSTATIC CONTROL IN A 3-ELECTRODE ELECTROCHROMIC DEVICE: TOWARDS BLACK TO TRANSMISSIVE AND MULTI-COLORED SWITCHING ........................................................................................... 75

4.1 Towards Black to Clear Switching ECDs -Fundamental Properties (SprayDOTTM-Purple 101, SprayDOTTM-Green 145, PProDOP-N-EtCN) ..................... 76 4.1.1 Film Deposition .......................................................................................................... 77 4.1.2 Polymer CV and Scan Rate Dependence .................................................................. 79 4.1.3 Spectroelectrochemistry ............................................................................................. 80 4.1.4 Setting Thicknesses .................................................................................................... 81 4.1.5 Tandem Chronocoulometry and Chronoabsorptometry .......................................... 84 4.1.6 Colorimetry ................................................................................................................. 86 4.1.7 Optical Stability .......................................................................................................... 88

4.2 SprayDOT-Purple 101/SprayDOT-Green 145 Dual-Film Electrochromic System ......... 89 4.3 Pseudo-Three-Electrode ECD (SprayDOTTM-Purple 101/SprayDOTTM-Green

145/PProDOP-N-EtCN)......................................................................................................... 92 4.4 Three-Electrode ECD (SprayDOT-Purple 101/SprayDOT-Green 145/PTMA) ............... 97 4.5 Conclusions ......................................................................................................................... 107

5 RGB COLOR SPACE 5-ELECTRODE ELECTROCHROMIC DISPLAY DEVICE ....... 109

5.1 RGB Color Space 5-Electrode ECD-Fundamental Properties (SprayDOTTM-Red 252, SprayDOTTM-Green 179, SprayDOTTM-Blue 153, PTMA) ..................................... 110

5.1.1 Film Deposition ........................................................................................................ 111 5.1.2 Polymer CV, Scan Rate Dependence ...................................................................... 112 5.1.3 Spectroelectrochemistry ........................................................................................... 114 5.1.4 Tandem Chronocoulometry and Chronoabsorptometry ........................................ 116 5.1.5 Colorimetry ............................................................................................................... 119

5.2 Dual Absorptive/Transmissive Window ECDs ................................................................ 122 5.3 RGB Color Space 5-Electrode ECD .................................................................................. 124 5.4 Conclusions and Future Perspectives ................................................................................ 127

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LIST OF REFERENCES ................................................................................................................. 129

BIOGRAPHICAL SKETCH ........................................................................................................... 134

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LIST OF TABLES

Table page 4-1 Coloration efficiencies and switch times of SprayDOTTM-Purple 101 and

SprayDOTTM-Green 145 at various film thicknesses in 0.1 M TBAP/PC. ........................ 86

5-1 Change in a*/b* values from a single EC film to multi-electrode devices. ..................... 127

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LIST OF FIGURES

Figure page 1-1 Doping mechanism of PEDOT.............................................................................................. 23

1-2 Spectroelectrochemical series of electrochemically deposited PEDOT film at applied potentials ................................................................................................................................. 25

1-3 Schematics of WO3 EC displays. ......................................................................................... 29

1-4 Schematic of a typical polymer dual type absorptive/transmissive window type ECD. ... 30

1-5 The luminosity of the human eye. ......................................................................................... 32

1-6 Calculation of CIE 1931 tristimulus values .......................................................................... 34

1-7 (a) CIE 1931 xy-chromaticity diagram, (b) 1976 CIE L*a*b* color space. .................... 36

1-8 Schematics of additive and subtractive color mixing system ............................................. 39

2-1 Setup for a standard two-probe surface resistivity measurement. ....................................... 50

3-1 Chemical structures of the polymers that are used in dual-polymer electrochromic method..................................................................................................................................... 58

3-2 The repeated potential scanning electropolymerization of EDOT ..................................... 59

3-3 The repeated potential scanning electropolymerization of ProDOP ................................... 59

3-4 Cyclic voltammograms of PEDOT in 0.1 M LiClO4/PC at different scan rates. ............... 60

3-5 Cyclic voltammograms of PProDOP in 0.1 M LiClO4/PC at different scan rates. ............ 61

3-6 Cyclic voltammograms of PProDOT-Hx2 in 0.1 M LiClO4/PC at different scan rates..... 61

3-7 Spectroelectrochemistry of PEDOT film in 0.1 M LiCLO4/PC solution. .......................... 63

3-8 Spectroelectrochemistry of PProDOP film in 0.1 M LiCLO4/PC solution. ....................... 63

3-9 Spectroelectrochemistry of PProDOT-Hx2 film in 0.1 M LiCLO4/PC solution. ............... 64

3-10 Tandem chronoabsorptometry and chronocoulometry experiments for PEDOT .............. 65

3-11 Tandem chronoabsorptometry and chronocoulometry experiments for PProDOP ........... 65

3-12 Tandem chronoabsorptometry and chronocoulometry experiments for PProDOT-Hx2........................................................................................................................................... 66

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3-13 Relative luminance as a function of applied potential of PEDOT ...................................... 68

3-14 Relative luminance as a function of applied potential of PProDOP .................................. 68

3-15 Relative luminance as a function of applied potential of PProDOT-Hx2 ........................... 69

3-16 UV–vis-NIR spectra of PProDOP/PEDOT from dual-polymer electrochromic setup...... 71

3-17 L*a*b* color coordinates and photography for PProDOP/PEDOT ................................... 72

3-18 L*a*b color coordinates and photography for PProDOP/PProDOT-Hx2 .......................... 73

4-1 Chemical structures of the polymers that are used in dual-polymer electrochromic method and in ECDs. ............................................................................................................. 77

4-2 Repeated potential scanning electropolymerization of ProDOP-N-EtCN.......................... 78

4-3 Cyclic voltammograms of PProDOP-N-EtCN in 0.1 M TBAP/PC at different scan rates ......................................................................................................................................... 79

4-4 Cyclic voltammograms of the SprayDOTTM-Purple 101 in 0.1 M TBAP/PC at different scan rates. ................................................................................................................ 79

4-5 Cyclic voltammograms of the SprayDOTTM-Green 145 in 0.1 M TBAP/PC at different scan rates ................................................................................................................. 80

4-6 Spectroelectrochemistry of SprayDOTTM-Purple 101 film. ............................................... 82

4-7 Spectroelectrochemistry of SprayDOTTM-Green 145 film. ................................................ 82

4-8 Spectroelectrochemistry of PProDOP-N-EtCN film. ......................................................... 83

4-9 Absorbance (a.u.) vs. thickness (Ao) linear fit plots for SprayDOTTM-Purple 101 and SprayDOTTM-Green. .............................................................................................................. 83

4-10 Tandem chronoabsorptometry and chronocoulometry experiment for SprayDOTTM-Purple 101(at 574 nm) ........................................................................................................... 85

4-11 Tandem chronoabsorptometry and chronocoulometry experiments for SprayDOTTM-Green 145 (at 465 nm) ........................................................................................................... 85

4-12 Tandem chronoabsorptometry and chronocoulometry experiments SprayDOTTM-Green 145 (at 707 nm) ........................................................................................................... 86

4-13 % Relative Luminance as a function of applied potential of SprayDOTTM-Purple 101 film at different thicknesses ........................................................................................... 87

4-14 % Relative Luminance as a function of applied potential of SprayDOTTM-Green 145 film at different thicknesses ................................................................................................... 88

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4-15 Electrochemical and optical stability of SprayDOTTM-Purple 101 and SprayDOTTM-Green 145................................................................................................................................ 89

4-16 UV–vis-NIR spectra of SprayDOTTM-Purple 101/SprayDOTTM-Green 145 from dual-polymer electrochromic setup ....................................................................................... 91

4-17 L*a*b* color coordinates and photography for SprayDOTTM-Purple 101/SprayDOTTM-Green 145. ............................................................................................... 92

4-18 Schematic of the P3-ECD under bipotentiostatic control .................................................... 94

4-19 UV–vis-NIR spectra of the P3-ECD. .................................................................................... 95

4-20 L*a*b* color coordinates and photography from the SprayDOTTM-Purple 101/SprayDOTTM-Green 145 P3-ECD. ................................................................................ 95

4-21 P3-ECD stability studies ........................................................................................................ 96

4-22 Schematic of the 3-ECD under bipotentiostatic control. ..................................................... 97

4-23 Schematic of the highly transmissive porous electrode (PETE/Au/PEDOT:PSS). ........... 99

4-24 Systematic % Transmittance study of counter electrodes/counter electrode components ........................................................................................................................... 100

4-25 Cyclic voltammograms of PTMA in 0.1 M TBAP/PC at different scan rates ................. 101

4-26 Redox couples of PTMA. .................................................................................................... 101

4-27 PTMA formulation studies in 0.1 M LiClO4/PC................................................................ 103

4-28 Spectroelectrochemistry of PTMA/PMMA.film................................................................ 104

4-29 UV–vis-NIR spectra of the 3-ECD. ................................................................................... 106

4-30 L*a*b* color coordinates and photography from the SprayDOT-Purple 101/SprayDOT-Green 145 3-ECD. ..................................................................................... 107

5-1 The schematic of the working principles of the 5-Electrode ECD. .................................. 111

5-2 Chemical structures of the polymers and the photographs of their neutral (N) and doped (D) states. ................................................................................................................... 111

5-3 Cyclic voltammograms of SprayDOTTM- Red 252 in 0.1 M TBAP/PC at different scan rates ............................................................................................................................... 112

5-4 Cyclic voltammograms of SprayDOTTM-Green 179 in 0.1 M TBAP/PC at different scan rates ............................................................................................................................... 113

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5-5 Cyclic voltammograms of SprayDOTTM-Blue 153 in 0.1 M TBAP/PC at different scan rates ............................................................................................................................... 113

5-6 Spectroelectrochemistry of SprayDOTTM-Red 252 film ................................................... 114

5-7 Spectroelectrochemistry of SprayDOTTM-Green 179 film ................................................ 115

5-8 Spectroelectrochemistry of SprayDOTTM-Blue 153 film. ................................................. 115

5-9 Tandem chronoabsorptometry and chronocoulometry experiment for SprayDOTTM-Red 252 ................................................................................................................................. 117

5-10 Tandem chronoabsorptometry and chronocoulometry experiment for SprayDOTTM-Green 179 (at 443 nm). ........................................................................................................ 117

5-11 Tandem chronoabsorptometry and chronocoulometry experiment for SprayDOTTM-Green 179 (at 634 nm) ......................................................................................................... 118

5-12 Tandem chronoabsorptometry and chronocoulometry experiment for SprayDOTTM-Blue 153 ................................................................................................................................ 118

5-13 Relative luminance as a function of applied potential and L*a*b* color coordinates and photography at redox extremes of SprayDOTTM-Red 252 in 0.1 M TBAP/PC. ....... 120

5-14 Relative luminance as a function of applied potential and L*a*b* color coordinates and photography at redox extremes of SprayDOTTM-Green 179 in 0.1 M TBAP/PC. ... 121

5-15 Relative luminance as a function of applied potential and L*a*b* color coordinates and photography at redox extremes of SprayDOTTM-Blue 153 in 0.1 M TBAP/PC. ..... 121

5-16 SprayDOTTM-Red 252/PTMA Window ECD. UV-vis-NIR spectra and the L*a*b* color coordinates with the associated photographs at the redox extremes. ...................... 123

5-17 SprayDOTTM-Green 179 /PTMA Window ECD. UV-vis-NIR spectra and the L*a*b* color coordinates with the associated photographs at the redox extremes. ........ 123

5-18 SprayDOTTM-Blue 153 /PTMA Window ECD. UV-vis-NIR spectra and the L*a*b* color coordinates with the associated photographs at the redox extremes. ...................... 124

5-19 Schematic of the RGB 5- ECD............................................................................................ 126

5-20 UV–vis-NIR spectra of the individual colors obtained from the RGB 5-ECD, (a) red, (b) green and (c) blue. .......................................................................................................... 126

5-21 L*a*b* color coordinates and photography of the RGB 5-ECD. ..................................... 127

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LIST OF ABBREVIATIONS

PAc polyacetylene

PANI polyaniline

VB Valence band

Eg bandgap energy

CB conduction band

CE coloration efficiency

PC Propylene carbonate

LiClO4 Lithium perchlorate

TBAP tetrabutylammonium perchlorate

CV Cyclic voltammetry

PMMA pol(methyl methacrylate)

PXDOT poly(3,4-alkylenedioxythiophene)

PXDOP poly(3,4-alkylenedioxypyrrole)

PTMA poly(2,2,6,6-tetramethylpiperidinyloxy-4yl methacrylate)

BTD 2,1,3-benzothiadiazole

PEDOT:PSS poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)

PETE polyester membrane

ECD electrochromic device

P3-ECD pseudo three electrode electrochromic device

3-ECD three-electrode electrochromic device

RGB 5-ECD RGB Color Space five-electrode electrochromic device

C luminance contrast

ΔE color contrast

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Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy

APPLICATION OF CONJUGATED POLYMERS TO MULTI-ELECTRODE

ELECTROCHROMIC DEVICES

By

Ece Unur

December 2008 Chair: John R. Reynolds Major: Chemistry

Electrochromism, change or bleaching of color with applied potential, is one of the most

eminent properties of conjugated polymers and it originates from electronic structure changes

induced upon redox doping/dedoping. Color tuning in conjugated polymers is possible by

synthetic and physical means. New colors can be accessed either by structural modifications that

allow the alteration of electronic properties (e.g. bandgap) or by newly developed analytical

methods/devices that utilize the optical properties of existing polymers. The absorption spectra

of the donor-acceptor based poly(3,4-alkylenedioxythiophene) (PXDOT) derivatives used in this

work spans the full visible spectrum in their neutral state, and bleach upon oxidation due to the

formation of lower energy states that are created at the expense of the HOMO-LUMO electronic

transitions. The dual polymer film technique, which is an analytical method derived from color

mixing theory, generates new colors by transmitting light through two films stacked together in

an electrolyte solution and under separate potentiostatic control. Here, we report on three new

multi-electrode electrochromic window devices, pseudo 3- electrode device (P3-ECD), 3-

electrode device (3-ECD) and RGB 5-electrode device (RGB 5-ECD), made possible by the dual

polymer film technique, comprising multiple active electrodes and non-color changing counter

electrodes. Having spray-processable RGB to transparent switching polymers available along

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with non color changing, yet electroactive, counter electrode polymers for the first time, multi-

electrode electrochromic devices under separate potentiostatic control promise a myriad of colors

by combining optical properties of two or more films.

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CHAPTER 1 ELECTROCHROMISM AND COLOR IN CONJUGATED POLYMERS

1.1 Introduction

The interest in conjugated conducting polymers as a new class of electronic materials

started with the Nobel Prize (Chemistry 2000) winning discovery. Researchers showed that the

chemical doping of polacetylene (PAc) films with electron accepting iodine vapor results in

seven orders of magnitude increase in conductivity.1 Chemical doping is a charge transfer redox

reaction, which can be conducted by exposing the material to a vapor or a solution of dopant.

The doping levels can be controlled by varying the exposure times and dopant concentration, but

still this method lacks the precise control and homogeneous distribution of dopants.2 Therefore,

electrochemical doping, in which the doping levels can be controlled by controlling the potential

applied between counter and working electrodes, was invented.3 In electrochemical doping,

redox charge is supplied to the conjugated polymer and ions diffuse into the polymer from the

electrolyte in order to maintain charge neutrality.2

Similar studies were applied to conjugated polymers with more complex structures, such as

polyaniline (PANI) which, by being stable in its conductive form, became an important polymer

for industrial applications.4 Another polymer with a complex structure, the conjugated

polyheterocyle polypyrrole (PPy), has found many applications. Dall’Olio et al. prepared the

first polypyrrrole by oxidative polymerization and it had a conductivity of 8 S/cm at room

temperature.5 Following that, Diaz et al. electropolymerized pyrrole to air stable freestanding

films.6 Polythiophene subsequently found more interest due to its higher stability in both doped

and neutral states and ease of 3- substitution which induces solubility.7, 8

Conjugated polymers became popular in electronic applications not because of their better

performance over inorganic semiconductors, but because of their unique combined properties.

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These neutral materials are charge transporting as semiconductors and have the physical and

mechanical properties of typical plastics. Electrochemical doping of conjugated polymers has

opened a new field with applications ranging from polymer batteries and supercapacitors,

electrochromic displays to redox sensors.7, 9 This introduction will cover evolution of color in

conjugated polymers and their electrochromic applications.

1.2 Electrochromism

The term electrochromism was first introduced by Platt, and demonstrated by Franz and

Keldysh.10 Electrochromism is the evolution of new optical absorption bands in an electroactive

species upon reversible electrochemical oxidation/reduction reaction.11-13 Due to the needs of

modern technology the definition of electrochromism is not limited to the visible region, but

extended to the ultraviolet (UV) and infrared (IR) regions of the electromagnetic spectrum.

Electrochromism in the visible region is useful for display purposes. There are three classes of

electrochromic materials; inorganic materials such as, transition metal oxides (e.g. tungsten

trioxide, WO3) and Prussian Blue, molecular electrochromes such as, viologens (4,4´-

bipyridylium salts) and conjugated polymers.9, 13-17

The mechanism of electrochromism, in other words electrochemical doping, in conjugated

polymers is different from the earliest electrochromes, alkali halides. There are two main

features to electrochromism in these crystals, first they must have F-centers, and second they

must constitute both ionic and electronic conductivity. F-centers, color centers, are

crystallographic defects arising from anion vacancy. Upon application of potential, these

vacancies fill with electrons and render a color to the material. The electrons migrating in to the

film from a near source can maintain the charge neutrality.10

Conjugated polymer electrochromism evolves from the emergence of lower energy

transitions between the valence (highest occupied π-electron) and the conduction (lowest

22

unoccupied) bands upon electrochemical doping. The energy difference between the conduction

and valence bands, termed the electronic bandgap (Eg), defines the intrinsic optical properties of

conjugated polymers. The π overlap of pz orbitals along the conjugated polymer backbone

allows the free movement of charge carriers. When an electron is withdrawn from the valence

band, a radical cation (known as a polaron), forms. The polymer chain partly gains a quinoid-

like geometry and thus new energy states. These half-filled new polaronic energy states are

distributed symmetrically in the electronic bandgap. Further oxidation of the polymer results

either in formation of new polarons upon withdrawal of electrons from different chains or

withdrawal of electrons from an existing polaron, which results in a dication called a bipolaron

and new energy states. (Figure 1-1) These charged defects along the polymer chain are

neutralized by the migration of counter anions into the polymer matrix, and this overall process

is called p-doping. The exit of concurrent anions results in reduction of ‘p-doped’ conjugated

conducting polymer to its neutral insulating form. The color change or contrast between the

doped and undoped forms of the polymer depends on the magnitude of the bandgap of the

undoped polymer. Thin films of conjugated conducting polymers with Eg greater than 3 eV

(~400 nm) are colorless and transparent in the undoped form, while in the doped form they

generally absorb in the visible region. Those with Eg equal to or less than 1.8-1.9 eV (~650-700

nm) tend to be highly absorbing in the undoped form but, after doping, the free carrier absorption

is relatively weak in the visible region as it is transferred to the near infrared (NIR). Polymers

with intermediate gaps have distinct optical changes throughout the visible region and can

exhibit several colors.

A spectroelectrochemical series obtained from a thin film of a conjugated polymer

distinctly elucidates the doping induced optical changes. As shown in Figure 1-2 cathodically-

23

coloring poly(3,4-ethylenedioxythiophene) (PEDOT), with a low bandgap of 1.6 eV shows an

intense π-π* absorption in the visible region with a maximum at 632 nm (2 eV) and appears blue.

Upon doping, a new absorption band emerges in the NIR region (~950 nm or 1.3 eV) due to the

formation of polarons while π-π* transition in the visible region diminishes. Upon further

doping, new polarons and bipolarons form and the optical intensity increases in the NIR region.

The tailing of the NIR absorption into the visible region gives the doped polymer a transmissive

sky blue appearance in the doped state.

Figure 1-1 Doping mechanism of PEDOT; neutral form, slightly doped radical cation (polaron) form and fully doped dication (bipolaron) form

1.3 Color Control in Conjugated Polymers

Conjugated polymers provide the ability to access various electrochromic states in both the

doped and neutral forms by controlling the band gap through structural modification of the

pendant groups or the conjugated backbone.18, 19 The range of colors obtained from conjugated

polymers based on thiophene and pyrrole moieties such as poly(3,4-alkylenedioxythiophene)

(PXDOT) and poly(3,4-alkylenedioxypyrrole) (PXDOP) together with their derivatives spans the

entire visible spectrum and extends into the UV and NIR regions.20 It has been shown that the

electrochromic contrast of these two polymer families can be enhanced by increasing the size of

VB / π

CB / π*

Eg

VB / π

CB / π*

VB / π

CB / π*

neutral polaron bipolaron

1

-e- -e-A-

A- A-

24

the alkylenedioxy ring, or the bulkiness of the substituent attached to the ring.21, 22 Dietrich et al.

first studied the electrochemical and optical properties of PEDOT and poly(3,4-

propylenedioxythiophene) (PProDOT).23. In the first systematic study of the PXDOTs, our

group reported on high optical contrasts and fast switching times for the di-alkyl substituted

(along the alkylene bridge) PProDOTs compared to the parent PProDOT.24 Di-substitution

results in a more open polymer morphology that enhances charge-compensating dopant ion

movement in and out of the matrix. Typically, a strong NIR absorption tailing into the visible

region evolves as the polymer becomes conductive. It is the attenuated tailing of this NIR

absorption as it crosses through the visible region that causes the higher transmissivity for the

PProDOT derivatives. ProDOT-Me2, substituted by dimethyl on the central carbon of the

propylene bridge, was shown to exhibit a high optical contrast of 78% (Δ%T) at 578 nm.22 More

highly substituted PXDOT derivatives, prepared by both electrochemical and solution

polymerization methods, exhibit high optical contrast, switching from deep red-purple to highly

transmissive sky blue where the human eye is highly sensitive, and fast switching times because

of the more open morphology. 25-29 Of these polymers, SprayDOTTM-Classic (PProDOT-

(CH2OEtHx)2) exhibits an especially high contrast 80% (Δ%T) at 581 nm26, 30 SprayDOT™s

have the advantage of being soluble in their reduced form in several organic solvents, allowing

the deposition of high quality films by spraying or spin-coating.19, 26, 31

On the other hand, with their blue shifted absorption, the PXDOPs exhibit larger bandgaps

compared to the PXDOTs. Poly(3,4-ethylenedioxypyrrole) (PEDOP) exhibits a bright red color

in its neutral state (bandgap 2.05 eV) and a light blue transmissive state upon oxidation while its

25

Figure 1-2 Spectroelectrochemical series of electrochemically deposited PEDOT film at applied potentials between -1.45 V and +0.35 V vs. Fc/Fc+ in 0.1 V increments. The photographs of the neutral and the doped films are shown in the inset.

thiophene counterpart PEDOT exhibits a blue color in its neutral state with a lower bandgap (1.6

eV). This difference is attributed to the higher LUMO of pyrroles relative to thiophenes. Upon

increasing the ring size of the alkylene bridge, even higher bandgaps are obtained (PProDOP, 2.2

eV vs. PProDOT, 1.7 eV).32 The neutral form of PProDOP has an orange color which bleaches

upon doping after passing through a brown color state arising from intermediate doping. Our

group has further increased the band gaps for PXDOPs by N-substitution. While inducing

torsion along the heterocyclic backbone, N-substitution decreases the π-conjugation and

therefore increases the band gap while maintaining the low oxidation potentials. Due to higher

band gaps (Eg > 3.0 eV), the π-π* transitions of the N-substituted PProDOPs blue shift into the

ultraviolet region and the intragap polaron and bipolaron transitions occur in the visible region.

1.0 1.5 2.0 2.5 3.0 3.50.0

0.5

1.0

1.5

2.0

1.6 eV

doped

Abs

orba

nce

(a.u

.)

Energy (eV)

Neutral

Ox

Red

Doped

π-π*

polaron

Bipolaron/polaron

26

As such, these materials are anodically coloring. 33 The nature of the substituent has an effect on

the extent to which the π-π* transition is shifted. For N-methyl PProDOP the bandgap occurs at

3.0 eV, compared to 2.2 eV for PProDOP. Both N-[2-(2-Ethoxy-ethoxy)-ethyl] PProDOP (N-

Gly PProDOP) and N-propanesulfonate PProDOP (N-PrS PProDOP) are colorless when fully

reduced and colored upon full oxidation. Walczak et al. invented a new synthetic route which

eases the laborious N- substituted 3,4-alkylenedioxypyrrole monomer synthesis by utilizing a

synthetic intermediate, an ester substituted dihydroxypyrrole.34, 35 Some of the highly

transmissive N-substituted PProDOPs, such as poly(3,4-propylenedioxythiophene-N-

propionitrile) (PProDOP-N-EtCN), have been shown to exhibit electroactivity without a change

of color.

One major focus in developing processable, high contrast EC polymers has been to

establish a full color palette for use in displays and printing. Red and blue polymers becoming

highly transmissive upon oxidation have been reported.13 But synthesis of green polymers which

absorb in the red and blue regions of the electromagnetic spectrum had been a challenge for

scientists until the first electrochemically stable donor-acceptor based green polymer was

reported by Sonmez et al.36 The green poly(2,3-di(thien-3-yl)-5,7-di(thien-2-yl)thieno[3,4-

b]pyrazine) PDDTP polymer obtained by oxidative electro-deposition was shown to switch to a

transmisive brown hue upon doping.37, 38 Toppare et al. reported on the first solution processable

green polymer, poly(2,3-bis(3,4-bis(decyloxy)phenyl)-5,8-bis(2,3-dihydrothieno[3,4-

b][1,4]dioxin-5-yl)quinoxaline) PDOPEQ, with a highly transmissive oxidized state.39 In

parallel our group reported on a set of green polymers with all intended properties and one of

them, poly(EDOT2(ProDOT-(CH2O(2-EtHx))2BTD) SprayDOTTM-Green 145, was used for the

electrochromic applications in this manuscript.40 The polymer was synthesized by alternative

27

addition of electron rich EDOT and 2-ethylhexyloxy-substituted 3,4-propylenedioxythiophene

ProDOT-(CH2O(2-EtHx))2 onto the strong acceptor 2,1,3-benzothiadiazole (BTD). The

fabrication of full-color electrochromic display devices ECDs is possible by the achievement of

materials that are processable and have the three primary colors, red, green and blue in their

neutral states and all can be convert to transmissive forms.37, 38, 41

1.4 Electrochromic Devices

The most widespread applications of electrochromic materials include rearview mirrors9,

protective eyewear9, displays20, smart windows11, optical shutters16, optical data storage11 and

electronic paper42. Conjugated polymers are finding growing utilization in electrochromic

applications due to their extensive cathodic, anodic and multi-color control, ease of

processability, flexibility, rapid redox switching, high optical contrast and long-term stability.

It has been shown that if a device under applied electric field exhibits changes in optical

properties (e.g. absorbance) in a reversible and controlled manner it could be utilized in displays

and information storage mediums. There has been a tremendous growth in display technologies

in the last decade; such as cathode ray tubes (CRT), liquid crystals (LCD), light emitting diodes

(LED), electrophoretic displays and interferometric modular displays (IMOD). Electrochromic

displays have been exploited by many researcher since they provide comparable characteristics

and unique features. Electrochromic displays consume little power, images persist for some time

due to the memory effect, a broad range in pixel size is possible, they provide long storage times,

wide viewing angles and ease of processing. Electrochromic devices are electrochemical cells

that modulate absorbed, transmitted, or reflected incident electromagnetic radiation upon

application of an electric field across the electrochromic materials within the device. It is also

convenient to think of the color and bleach process as the charging and discharging of a battery.

28

Deb introduced the first electrochromic display in 1969 and the schematic of two ECDs

proposed are illustrated in Figure 1-3.43, 44 In the first device a thin layer of inorganic

electrochrome, tungsten oxide (WO3), was evaporated on a transparent conductive substrate

(NESA glass) and this was assigned as the working electrode (cathode). An insulating layer was

deposited on tungsten oxide film. The transparent counter electrode (glass substrate with a thin

layer of gold) was closed on top of the insulating layer to complete a solid state capacitor

structure. When the NESA and gold electrodes are biased, electrons are injected through the

cathode into the transparent tungsten oxide film and the film becomes deep blue. When a

reverse bias is applied, Au becomes the new cathode and the NESA glass becomes the new

anode. The insulating layer prevents the electron injection from the Au cathode in to the WO3

film. The new anode pulls the previously injected electrons in the film back and the film returns

to its original transmissive state. In the second type of device, the insulating layer was replaced

with an acidic electrolyte, which conducts protons but electrons. The device was based on the

double injection of electrons and protons in to the material. In this device the charges of

electrons that were injected into the WO3 layer by the cathode (indium tin oxide) are

counterbalanced by the protons that were supplied from the electrolyte and tungsten bronze

(HxWO3, x~0.5) forms at the cathode. When the bias between ITO and carbon is reversed

electrons leave at the anode, protons leave at the cathode, and the film is bleached. In this type

of device, a high ion mobility is required since the coloration entails injection of positive ions

and bleaching entails extraction of positive ions.

Organic ECDs were first presented by Schoot et al.45 In that device an aqueous solution of

heptyl viologen bromide was used as an active layer in between two transparent electrodes and

aqueous potassium bromide (KBr) was used as an electrolyte. Upon the application of potential

29

𝑊𝑊𝑊𝑊3 (𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑟𝑟) + 𝑥𝑥𝑐𝑐− + 𝑥𝑥𝑀𝑀+ ↔ 𝑀𝑀𝑥𝑥𝑊𝑊𝑊𝑊3 (𝑏𝑏𝑐𝑐𝑏𝑏𝑐𝑐)

Figure 1-3 Schematics of WO3 EC displays

heptyl viologens dication reduces to purple colored radical cation at the cathode. This reduction

is followed by a reaction between radical cation and bromide ions to form an insoluble purple

solid (λmax=545 nm) at the cathode which bleaches when the bias is reversed. This highly stable

device (105 redox cycles) exhibits 20% reflectance contrast. 44, 45

ECDs have found utilization in conjugated conducting polymer applications. Other than

the absorptive/transmissive-type ECD in which both working and counter electrodes are

transparent so that light can pass through, reflective ECDs have also been developed. In

reflective ECDs the active electrochromic polymer is deposited onto an outward-facing reflective

electrode, such as gold deposited onto a flexible, ion permeable substrate.46 In this work, we

have constructed new types of absorptive/transmissive window type ECDs The schematic of an

absorption/transmission window type ECD which is used in polymeric applications is given in

Figure 1-4. The device switches from a colored to transmissive state and is composed of a

working electrode with an active layer and a counter electrode separated by a gel electrolyte.

The dual polymer electrochromic device design constitutes the use of a second electrochromic

material on the counter electrode to balance the reaction in the working electrode and prevent

CE WE

glass substrate

Transparent/conductiveLayer (NESA)

WO3 Insulating layer

Thin layer of Au

CE

Liquid electrolyte

WE

glass substrate

Transparent/conductiveLayer (ITO)

WO3

carbon layer

stainless steel

30

early degradation of the functional material. Unfortunately, the use of these devices is limited to

electrochromic materials with complementary optical properties, one anodically coloring and the

other cathodically coloring.30, 47 Recently Otero and Padilla have reported that the contrast

achieved in dual electrochromic systems could not exceed the contrast obtained from a single

system. Higher contrast of dual electrochromic windows is possible only through the careful

design and use of an electroactive yet non-electrochromic highly transmissive polymers as a

counter electrode material.48 Anodically coloring ProDOP derivatives are strong candidates for

use as counter electrode materials due to their poorly saturated colors.

Figure 1-4 Schematic of a typical polymer dual type absorptive/transmissive window type ECD

1.5 Color and Colorimetry

The perceived color of an object depends on the wavelengths of the electromagnetic

spectrum it reflects (transmits)/absorbs, under which light source it is being observed and the

response of the observer. Since color is a perceptual property, it needs a quantitative definition.

Colorimetry is a term used for the quantitative description of colors as they appear to the human

eye.49

By the twentieth century, it was accepted that there are two fundamental ways to describe

colors quantitatively. The first system is spectra, which doesn’t take into account any vision

CE WE

glass substrate

ITOAnodically coloring EC polymer

Gel electrolyteCathodically coloring EC polymer

CE WE

glass substrate

ITOAnodically coloring EC polymer

Gel electrolyteCathodically coloring EC polymer

31

factors. The second system is based on the physiological properties (the human eye response to

visible light of various wavelengths and intensities). The CIE-Commission Internationale de

l’Eclairage system that will be detailed later in this section is the most widely used system.

In order to quantify color, every component responsible for the perception, light source,

reflectance from the object and human eye response, must be quantified first. The sensitivity of

human eye over the visible range under illumination is called photopic luminosity while it is

called scotopic luminosity in the dark. There are two types of photoreceptors in the human eye,

cone shaped and rod shaped receptors. Three different types of cone shaped receptors function

under illuminated conditions and they are responsible for red, green, and blue trichromatic (more

accurately long-, medium- and short-wavelength) sensation, separately. The rod shaped

receptors function in the dark and they are responsible for the night vision (monochromatic

vision). The photopic luminosity reaches a maximum at 555 nm while the scotopic luminosity

reaches a maximum at 500 nm and they both decrease down to zero at 400 and 700 nm. As

shown by Figure 1-5 the human eye is most sensitive to wavelengths around 555 nm during the

day and 500 nm at night. In addition to human eye sensitivity, the color of an object also

depends on the light source. Objects with different reflectance spectra may appear to have the

same color under one light source, and appear different under another one. The dependence of

color on the light source is called metamerism. If the color appears the same under a wide

range of light sources it is, then, called color constancy.50-54

The first mathematically standardized color space, CIE 1931 XYZ, was introduced by the

International Commission on Illumination (CIE). The system is based on quantifying the

visual stimulant and the trichromatic response of the human eye to this stimulant. The visual

32

Figure 1-5 The luminosity of the human eye. (Plots are derived from spectral data distributed by CIE Technical report.55)

stimulant is the combined effect of a light source and the object being observed under that light

source. A light from a physical source appears white and can be dispersed in to wavelengths by

a prism as shown by Newton. An illuminant is a plot of relative energy distribution of a light

source over the entire visible spectrum. In other words, illuminants are used to quantify physical

light sources. Power distribution curves (S (λ)) of some common CIE illuminants are given in

Figure 1-6 (a). Illuminant A simulates incandescent light, illuminant C simulates average

daylight, illuminants D 65 and D 50 simulate natural daylight at different temperatures. In our

labs, to date we have used illuminant D50 for Colorimetry experiments. The colorants in objects

modify light by absorption, transmission or reflection. The amount of reflected or transmitted

light from an object can be can be quantified by a spectrophotometer. A transmittance spectrum

(T (λ)) of a potentiostatically deposited PEDOT film at its neutral state is shown in Figure 1-6

(b). The last step in quantification of color, the quantification of human eye perception of light,

was accomplished after series of experiments. Observers were sat in front of a screen with a 2-

400 450 500 550 600 650 700 750

0.0

0.2

0.4

0.6

0.8

1.0 555 nm

Rel

ativ

e R

espo

nse

wavelength (nm)

scotopic photopic

500 nm

33

degree viewing aperture. Half of the screen was illuminated. Subjects were asked to match that

light by adjusting red , green and blue lights . This procedure was repeated for all the colors in the

visible spectrum . The functions that quantify the red ( ͞x (λ), green ( ͞y (λ) ) and blue ( ͞z (λ))

sensitivity of human eye were derived and they were called CIE 1931 20 standard observer

(Figure 1-6 (c)). In 1964, 10-degree viewing standard observer was defined due to the findings

that cones are more spread at the back of the eye meaning that human eye has a wider vision.

The quantified data for the source, transmitted/reflected light from the object and observer

perception (standard observer) can be multiplied at every wavelength over the entire visible

spectrum and then summed to obtain CIE X Y Z tristimulus values of the color. The process can

also be summarized by the following equations.49, 54, 56

𝑋𝑋 = 100

∫ 𝑆𝑆(𝜆𝜆)𝑦𝑦𝑦(𝜆𝜆)𝑑𝑑𝜆𝜆𝜆𝜆2𝜆𝜆1

∫ 𝑆𝑆(𝜆𝜆)𝑇𝑇(𝜆𝜆)𝑥𝑥𝑦(𝜆𝜆)𝑑𝑑𝜆𝜆𝜆𝜆2𝜆𝜆1

𝑌𝑌 = 100

∫ 𝑆𝑆(𝜆𝜆)𝑦𝑦𝑦(𝜆𝜆)𝑑𝑑𝜆𝜆𝜆𝜆2𝜆𝜆1

∫ 𝑆𝑆(𝜆𝜆)𝑇𝑇(𝜆𝜆)𝑦𝑦𝑦(𝜆𝜆)𝑑𝑑𝜆𝜆𝜆𝜆2𝜆𝜆1

𝑍𝑍 = 100

∫ 𝑆𝑆(𝜆𝜆)𝑦𝑦𝑦(𝜆𝜆)𝑑𝑑𝜆𝜆𝜆𝜆2𝜆𝜆1

∫ 𝑆𝑆(𝜆𝜆)𝑇𝑇(𝜆𝜆)𝑧𝑧𝑦(𝜆𝜆)𝑑𝑑𝜆𝜆𝜆𝜆2𝜆𝜆1

The factor 100

∫ 𝑆𝑆(𝜆𝜆)𝑦𝑦𝑦(𝜆𝜆)𝑑𝑑𝜆𝜆𝜆𝜆2𝜆𝜆1

was used to assign the value Y= 100 to a perfect reflecting diffuser

or to a perfect transmitter. 49 We have obtained the CIE 1931 tristimulus values for neutral

PEDOT film as X= 10, Y= 9.8 and Z= 33.6 from integration of curves in Figure 1-6.

The tristimulus values (the amount of red, green and blue primaries perceived) are used to

map the color on 3D vector space. In addition, Y is arranged to correspond exactly to the

average luminous curve for an average eye, thus, it is a direct measure of luminosity.

34

Figure 1-6 Calculation of CIE 1931 tristimulus values, (a) Relative power distributions of CIE illuminants A, C, D65 and D50,( b) Transmittance of PEDOT film on ITO/glass electrode at -1.35 V vs. Fc/Fc in 0.1 M LiClO4/PC, (c) CIE 1931 2-degree observer (color matching functions), (d) CIE 1931 XYZ tristimulus values for PEDOT film, (e) Tristimulus values, chromaticity coordinates and a photograph of PEDOT film. (Results are derived from spectral data distributed by CIE Technical report.55)

400 450 500 550 600 650 700 7500.0

0.5

1.0

1.5

2.0

z

y

Sens

itivi

ty

wavelength (nm)

x

c

400 450 500 550 600 650 700 750

0

10

20

30

40

50

60

70

Inte

nsity

wavelength (nm)

X Y Z

d

TristimulusX = 10Y = 9.8Z = 33.6

Chromaticityx = 0.187y = 0.183

e

400 450 500 550 600 650 700 7500

50

100

150

200

250

R

elat

ive

Pow

er (S

(λ))

wavelenght (nm)

A-incandescent D65-daylight at 6500 K C-average daylight D50-daylight at 5000 K

a

400 450 500 550 600 650 700 7500.0

0.2

0.4

0.6

0.8

1.0

Tran

smitt

ance

wavelength (nm)

b

35

In order to be able to represent color on 2D space, new quantities called the chromaticity

coordinates, x, y, z were derived.

Given that,

𝑥𝑥 + 𝑦𝑦 + 𝑧𝑧 = 1

𝑥𝑥 = 𝑋𝑋𝑋𝑋+𝑌𝑌+𝑍𝑍

𝑦𝑦 = 𝑌𝑌𝑋𝑋+𝑌𝑌+𝑍𝑍

𝑧𝑧 = 𝑍𝑍𝑋𝑋+𝑌𝑌+𝑍𝑍

If luminosity is disregarded, x and y alone are sufficient to describe a color. In the CIE

chromaticity diagram, x is the abscissa and y is the ordinate as shown in Figure 1-7 (a). The

colors of the spectrum (monochromatic) lie on the parabolic curve. Achromatic colors lie on a

line perpendicular to that color plane. The dominant wavelength is given by the point at which a

straight line drawn from the white point (W) through color point (P) intersects the spectral curve.

The xy-chromaticity coordinates of the PEDOT film were calculates from its tristimulus values

as x= 0.183 and y= 0.187 which corresponds to the point P in the blue region of the chromaticity

diagram (Figure 1-7 (a)). Purity, p, is the ratio of distance between W and P to distance between

W and dominant wavelength. The purity of the PEDOT film is 64%. The nearer the is color to

the spectral curve the purer it is. Extrapolation of the straight line to the opposite end gives the

dominant wavelength of the complementary color. Complementary colored lights add up to

white light. The dominant wavelength for PEDOT is 480 while the dominant wavelength of its

complementary color is 575 nm. Luminosity, dominant wavelength and purity constitute three

main characteristics of color, brightness, hue and saturation.17, 54

36

Figure 1-7 (a) CIE 1931 xy-chromaticity diagram, (b) 1976 CIE L*a*b* color space

Colorimetry has been utilized to precisely map the colors associated with samples.

Colorimetry experiments can be done with a portable colorimeter under a specified light source

or by a colorimetric spectrophotometer. Colorimeters have red, green and blue filters. Light

transmitted from the object passes through each filter, is detected separately, and the CIE XYZ

values are determined. Variations in color measurements are due primarily to differences

between the photodetector-filter spectral response and that of the1931 CIE standard observer.

Another source of error is variation in the intensity of the illumination source. 57 As an example

of that the D50 illuminant we utilize in our labs introduces an error of 5% in x-chromaticity

coordinate and 7% in y-chromaticity coordinate. (for CIE 1931 2-degree observer, CIE D50

Illuminant (x= 0.346, y= 0.358), D50 illuminant in our labs (x= 0.363, y= 0.386))

Colorimetric spectrophotometers break the light that is reflected from the object into

wavelengths and the intensity at each wavelength is recorded. The spectral data then is

multiplied by selected illuminant and standard observer data. Spectrophotometers are typically

used in high-precision color measurement applications and provide the greatest accuracy of the

different types of color measurement systems.57

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

R (575 nm)

Q

.y

x

.W 700 nm

400 nm

.P λD

.

aL*=100

L*= 0

b

37

In 1976, CIE recommended the LAB system as standard (CIELAB or CIE L*a*b*), in

order to provide standard and uniform color space that allows comparison of color values. In

this system L* represents lightness, a* and b* represents hue and chroma. The minimum L* is

zero representing black and maximum L* is 100 representing a perfect reflecting diffuser.

Positive a* represents red and negative a* is green, while positive b* is yellow and negative b* is

blue. These values have no numerical limits. (Figure 1-7 (b)). This system has the advantage

that colors are equally spaced and the differences between the points plotted on that color space

correspond to visual differences between the colors. 53 Color coordinates can be recorded as Y

(Lv) (photometric luminance with units of cd/m2) and xy (the chromaticity coordinates), then,

these values are converted tristimulus values. X, Y, Z tristimulus values can be converted to L*,

a* and b* coordinates of CIE 1976 (L*, a*, b*) color space (CIELAB) by the following

equations, where Xn, Yn and Zn are the tristimulus values for the standard illuminant.49, 55, 56

𝐿𝐿∗ = 116 � 𝑌𝑌𝑌𝑌𝑛𝑛�

13 − 16

𝑐𝑐∗ = 500 �� 𝑋𝑋𝑋𝑋𝑛𝑛�

13 − � 𝑌𝑌

𝑌𝑌𝑛𝑛�

13�

𝑏𝑏∗ = 200 �� 𝑌𝑌𝑌𝑌𝑛𝑛�

1/3− � 𝑍𝑍

𝑍𝑍𝑛𝑛�

1/3�

Where X/Xn, Y/Yn and Z/Zn are all greater than 0.008856.

The total color, Q, which is a magnitude of 3D vector in a color space or the distance from the

origin, can be represented as follows;

𝑄𝑄 = �(𝐿𝐿 ∗2+ 𝑐𝑐 ∗2+ 𝑏𝑏 ∗2)

The color difference, which is the magnitude of a vector between two specified points in color

space can be calculated as follows.

38

∆𝐸𝐸𝑐𝑐𝑏𝑏∗ = �(∆𝐿𝐿 ∗2) + (∆𝑐𝑐 ∗2) + (∆𝑏𝑏 ∗2)

where ΔL*, Δa* and Δb* represent changes in L*, a*, b* (throughout this work it represents the

difference between the colored and the bleached state of the film). 55 Color difference is used in

industry for matching colors of the products to the standards set.

1.6 Color Mixing Theory

There are two color systems that match a humans’ trichromatic vision; the additive color

system and subtractive color system. The primaries for the additive color system are red, green,

blue (RGB) while the primaries for the subtractive color system are cyan, magenta, and yellow

(CMY). In the additive color system, different colored light at different intensities add to make a

new color and this system is utilized in TV screens and monitors. In the subtractive color

system, an object subtracts certain wavelengths from white light by absorption and transmits or

reflects the rest. Subtractive color mixing system is utilized in dyes, pigments and in printing. In

Figure 1-8, the large circles on the corners of the large triangles represent primary colors, colors

that cannot be obtained by mixing other colors in their system. Small circles on the dashed

triangles represent secondary colors; colors obtained from mixtures the of primary colors. Equal

intensities of the three primary colors (red, green and blue) add to white light in the additive

system. Equal intensities of pigments of three primary colors (cyan, magenta and yellow) block

all the wavelengths and the object appears black in the subtractive system. As shown in Figure

1-8, when white light strikes the magenta cube, first the subtractive principle causes the dye

subtract/absorb green and reflect red and blue. Then the reflected red and blue light combine to

produce magenta with respect to additive principles.55, 56

39

Figure 1-8 Schematics of (a) additive color mixing system, (b) subtractive color mixing systems, (c) additive color mixing, (d) color generation from a reflective object and (e) color generation from a transmissive object.

In this dissertation, the combined effect of subtractive and additive color mixing was

utilized in the development of multicolor electrochromic displays. Electrochromic films that are

deposited on transparent electrodes were used as color filters. Films were illuminated by D50

white light at the back and observed from the front. As in Figure 1-8, when the light is

transmitted through the magenta film, the green portion is absorbed and the red and blue portions

are transmitted. Then the blue and red wavelengths (lights) add to magenta.

M

YC G

RB

CMYK

R

GB C

YM

RGB

Additive

Additive

Subtractive

white light

a b

dc

e

40

1.7 Structure of Dissertation

The focus of this work is the electrochemical and optical characterization of conjugated

polymers and their application to newly developed multi-electrode electrochromic devices.

Chapter 2 briefly summarizes the optical and electrochemical characterization methods for

electrochromic polymers. The details for the device constructions are given in the associated

chapters.

A new dual-polymer electrochromic film characterization method using bipotentiostat

control is introduced in Chapter 3. This method, which depends on the separate control of

doping levels in multiple electrochromic films, allows color mixing by physical means.

Chapter 4 utilizes the dual-polymer electrochromic film characterization technique in the

development of two new multi-electrode electrochromic devices; the Pseudo-3 Electrode

Electrochromic Device (P3-ECD) and 3-Electrode Electrochromic Device (3-ECD). The

working principles of the device is similiar to the dual-polymer electrochromic film

characterization method in that the doping levels of the electrochromic components are

independently controlled. These devices exhibit switches from black to transmissive upon color

mixing of green and purple absorbing electrochromic components. A new highly transmissive,

conductive and porous electrode material is introduced and used to serve as a counter electrode

to the electrochromic components of the 3-ECD.

In Chapter 5 a new RGB 5-Electrode electrochromic device (RGB 5-ECD) is elucidated.

The device constitutes 3 electrochromic films with primary colors red, green and blue, that

switch to transmissive upon doping. Independent control of doping levels, thus colors of

electrochromic films allows the establishment of an RGB color space electrochromic device.

41

CHAPTER 2 EXPERIMENTAL TECHNIQUES

The intent of this chapter is to provide an overview of the materials, techniques, and

instrumentation used during the course of this research. Complete characterization methods of

conjugated electroactive polymers and their applications to various devices explained here will

be detailed and frequently referred to in the subsequent chapters.

2.1 Chemicals, Materials and Instrumentation

Propylene carbonate (PC) was obtained from Acros Organics, lithium perchlorate (LiClO4)

and poly(methyl methacrylate) (PMMA, Mw 996,000 g/mol) were obtained from Aldrich and

they were all used as received. Tetrabutylammonium perchlorate (TBAP) was prepared by

mixing a 1:1 mole ratio of tetrabutylammonium bromide dissolved in water with perchloric acid.

The precipitate was filtered, recrystallized from a 1:1 molar ratio ethanol and water and dried in

the vacuum oven for 24 hours at 60°C. Ferrocene (Fe(C5H5)2) was obtained from Fluka. The

monomer, 3,4-ethylenedioxythiophene (EDOT) (Baytron M V2) was provided by H.C. Starck

and distilled under vacuum from CaH2. The monomer, 3,4-propylene dioxythiophene (ProDOP)

and poly(2,2,6,6-tetramethylpiperidinyloxy-4yl methacrylate) (PTMA) were provided by CIBA

Specialty Chemicals. Poly(3,3-dihexyl-3,4-dihydro-2Hthieno[3,4-b][1,4]dioxepine) (PProDOT-

Hx2) (Mw 66,000 g/mol, PDI 1.7) was synthesized as previously described.26 Poly(3,4-

propylenedioxythiophene-N-propionitrile) (PProDOP-N-EtCN), SprayDOT-Green 145 (Mw

12,600 g/mol, PDI 1.4), SprayDOT-Purple 101 (Mw 84,900 g/mol, PDI 2.1), SprayDOT-Red

252 (Mw 17,900 g/mol, PDI 2.2), SprayDOT-Green 179 (Mw 88,600 g/mol, PDI 2.1) and

SprayDOT-Blue 153 (Mw 42,700 g/mol, PDI 2.6) were synthesized by Reynolds’ Group.40, 58, 59

All polymer solutions were filtered through 0.45 µm Whatman Teflon (PTFE) syringe filters

prior to spraying. Formulated aqueous dispersion of poly(3,4-

42

ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), Clevios™ PH 500, was

purchased from H.C.Starck. (resistivity = 0.1 ohm-cm, bulk conductivity = 500 S/cm, mean

particle size = 30 nm, solids content = 1.2%). The formulation requires prior addition of 5 w %

dipolar organic solvent such as DMSO in order to achieve high conductivity films.

ITO-coated polished float glass slides CG-51IN-CUV (7 × 50 × 0.7 mm, Rs= 8-12 Ω) and

CG-51IN-S107 (25×75×0.7 mm Rs = 8-12 Ω) were obtained from Delta Technologies, Ltd. ITO

coated glass slides were wiped off with acetone to remove the immediate residue and air dried

prior to use. Contacts to the ITO slides were made using conductive Cu tape (1131) purchased

from 3M. Gass beads (100 micron) were obtained from BioSpec Products, Inc. and washed with

acetone and air dried before use. Track etched polyester (PETE) membranes (PETI00SP, 20 ×

25 cm sheets) were purchased from Sterlitech Corporation. Membranes were 9 µm thick with 10

μm diameter cylindrical pores. The membrane has a pore density of 105 pores/cm2 and is

resistant to organic solvents such as acetonitrile (ACN), propylene carbonate (PC) and dimethyl

sulfoxide (DMSO). Pure gold coins (99.99%) were purchased from National Coin Investors,

Inc. (Gainesville, FL) and cut into pieces for metal deposition. Platinum wire and sheets and

silver wire were purchased from Alfa Aesar. Platinum button electrodes and electrode polishing

kit were purchased from BASi (www.bioanalytical.com). Spectrosil® Quartz cuvettes (1/Q/1)

with a useable range of 170 to 2700 nm and with a 10 mm path length were obtained from

Starna Cells. The potentials for fundamental electrochemical studies, spectroelectrochemistry,

tandem chronoabsorptometry/chronocoulometry and devices were controlled by EG&G PAR

model 273A potentiostat/galvanostats (controlled using CorrWare software (Scribner

Associates)) in a three-electrode cell configuration. The potential in multi-electrode systems was

controlled by Pine Bipotentiostat model AFCBP1. When performing two electrode studies with

43

the standard potentisostats the reference and the counter electrodes were shorted together as a

single counter electrode. All absorption/transmission, spectroelectrochemistry, and

chronoabsorptometry experiments were carried out on a Varian Cary 500 Scan UV–vis-NIR

spectrophotometer. Immediate absorbance measurements at λmax in order to control the thickness

of films while spraying were done by Genesys 20 Spectrophotometer (Thermo Electron

Corporation). Colorimetry was carried out using a Minolta CS-100 Chroma Meter. Digital

photographs were taken with a Fujifilm FinePix S7000 digital camera by back illumination with

a D50 (5000K) light source. Veeco Dektak 150 Profilometer was used for thickness

measurements (UF Nanofabrication Facility). Airbrush, iwata-eclipse HP-BS (purchased from

www.iwata-medea.com), was used for spraying films. Keithley 2400 source meter was used for

two-probe surface resistivity measurements.

2.2 Electrochemistry

2.2.1 Electrochemical Setup

Electrochemical measurements were performed in a three-electrode cell configuration via

potentiostatic or potentiodynamic methods. In this set-up the potential is applied to the system

through working and reference electrode. The three-electrode cell configuration eliminates the

potential drop error that arises because of the resistance of solution and contacts. The current

response is recorded through working and counter electrodes. The glass cylindrical cell was

filled with 0.1 M electrolyte/solvent couple and the working, counter and reference electrodes

were immersed in the solution. Components of the electrolyte solution (salt and solvent) and

electrode materials must be chosen to ensure wide electrochemical window. Electrode materials

must be highly conductive. The Pt-button working electrode with a surface area of 0.02 cm2 was

polished prior to use, as detailed below, in order to remove the immediate adsorbed material.

The platinum flag or Pt wire counter electrode with a surface area greater than or equal to the

44

surface area of the working electrode was flamed before use to remove any residue. A pseudo

reference Ag wire electrode was used in this work and was calibrated vs. Fc/Fc+ by using 5 mM

ferrocene/0.1 M electrolyte solution. Before and after each experiment, the cell was filled with

the ferrocene solution and the potential range was scanned two times. Anodic and cathodic peak

potentials (Epa and Epc) for Fe+2/Fe+3 redox reaction (also denoted as Fc/Fc+) were determined

from these CV scans. The average of these peak potentials gives the half-wave potential

(𝐸𝐸1/2 = 𝐸𝐸𝑝𝑝𝑐𝑐 +𝐸𝐸𝑝𝑝𝑐𝑐2

) of the Fc/Fc+ against the silver wire. The E1/2 value was subtracted from the

experimental potentials in order to convert them from vs. Ag wire to vs. Fc/Fc+. Since the Ag

wire has a high impedance the current passing through tends to shift the potential noticeably

during the experiment. Therefore, calibrations must be done constantly. The solution was

bubbled with and inert gas (Ar) for 10 minutes before starting the experiment to remove oxygen

which can react (reduce). As it has been extensively reviewed by a previous researcher in the

Reynolds’ Group and Pavlishchuk et al., the Fc/Fc+

redox couple will be considered 5.1 eV

below the vacuum in this work.60, 61

2.2.1.1 Pt-button electrode preparation

The polishing kit, obtained from BASi, consist of two disk pads, a brown velvety Texmet

pad and a white nylon pad. After dampening the disk pads with distilled water, a few drops of

the 1-μm diamond polish and a few drops of alumina suspension were added to the nylon and

Texmet pads, respectively. The electrode surface was placed on the nylon pad and polished

using circular motions. The same procedure was followed on the Texmet pad. Then the

electrode was rinsed with methanol or acetone.62

45

2.2.2 Electrochromic Polymer Film Formation

Polymer films can be electrochemically deposited from their monomers. If we have

soluble EC polymers they can be drop cast on small surfaces such as Pt-button electrodes and

spray cast on large surfaces such as ITO/glass electrodes.

2.2.2.1 Electrochemical deposition

Electrochemical polymerization to form an electroactive and electorchromic polymer films

was carried out in a 0.1 M electrolyte, 10 mM monomer, solution potentostatically,

galvanostatically or potentiodynamically. For electro-deposition of a polymer film on a Pt button

(0.02 cm2) electrode, the three-electrode cell with a platinum flag counter electrode, and Ag/Ag+

reference electrode was used. For electro-deposition of a polymer film onto ITO/glass electrode

(2.27 cm2 active film area) the three-electrode cell configuration in a quartz cell with a platinum

wire counter electrode, and a Ag wire reference electrode was used. Higher surface resistivities

and larger surface areas as compared to of button electrodes ITO/glass electrodes (8-4 ohms)

results in high –IR drops across the film and gives broader voltammetric peaks.

2.2.2.2 Spray or drop casting

The soluble polymer films were drop cast from their neutral form on Pt-button electrodes

for fundamental electrochemical studies. For optical studies polymers were spray-coated onto

ITO-coated glass slides using an airbrush at 20 psi Argon pressure. Polymers were sprayed from

a solution of polymer in an appropriate solvent such as toluene which has a low vapor pressure.

The solution concentrations of 1-5 mg polymer/1 mL solvent were used. Polymer films were

then air dried. (The detailed information about each polymer film is given in the associated

chapter.) The thicknesses of the films were controlled by monitoring the absorbances at λmax of

each polymer film while spraying. (In Chapter 4, for each polymer, a set of films with various

46

absorbances (at λmax) was prepared and thicknesses were measured. The absorbance versus

thickness plots provided the linear calibration equations.)

2.3 Electrochromic Film Characterizations

2.3.1 Spectroelectrochemistry

Spectroelectrochemistry plays the main role in examining the optical changes that occur

upon doping or dedoping of electrochromic films and devices. It provides information about the

polymer’s band gap and intraband states created upon doping.

For spectroelectrochemical analysis, a quartz cell which utilizes a Ag wire pseudo

reference electrode, a Pt wire counter electrode and ITO/glass working electrode was used. In a

typical experiment, 0.1 M electrolyte solution and a blank ITO slide were placed in both the

sample and the blank cuvettes. Solutions in both cuvettes were degassed with argon for 10

minutes prior to background collection. The background was collected over the range of 1600

nm to 350 nm every 1 nm. Spectral data could be converted to eV by the equation given below.

It must be kept in mind that eV plot will highly compress the data especially near IR region

(many points in a very small range).

𝐸𝐸( 𝐽𝐽 ) = ℎ×𝑐𝑐𝜆𝜆

= 6.626×10−34 𝐽𝐽 .𝑠𝑠×2.99×108𝑚𝑚 .𝑠𝑠−1

𝜆𝜆(𝑛𝑛𝑚𝑚 )×10−9𝑚𝑚 .𝑛𝑛𝑚𝑚−1

𝐸𝐸(𝑐𝑐𝑒𝑒) = 1.98×10−16𝐽𝐽𝜆𝜆(𝑛𝑛𝑚𝑚 ) × 1 𝑐𝑐𝑒𝑒

1.602 ×10−19𝐶𝐶×1𝑒𝑒= 1240 𝑐𝑐𝑒𝑒

𝜆𝜆(𝑛𝑛𝑚𝑚 )

After background collection, the polymer coated ITO/glass was placed in the sample

cuvette and leads were attached. The polymer film was conditioned by sweeping the potential

between the oxidized and neutral potentials predetermined by CV studies on a Pt-button

electrode and IT0/glass electrode for 20 cycles. Most of the polymers in this work were stable in

their neutral state therefore the scan was started from the neutral potential. While the potential

was held where the polymer is neutral the energy was scanned from 1600 nm (0.775 eV) near-IR

47

to 350 nm (3.54 eV) uv-region. Then, the potential was stepped in 50 mV or 100 mV increments

until the completely oxidized states were reached.

2.3.1.1 Dual method

Two blank ITO/glass slides were stacked together back –to-back and inserted into an

electrolyte solution in a quartz cell to be used as a blank. One blank was placed in the sample

compartment while the other one was placed in the reference compartment. The background was

collected. The electrochromic polymer was replaced with the blank in the sample compartment.

The absorbance spectra were monitored at applied potentials.

2.3.1.2 Electrochromic Devices

Blank devices, ITO-coated glass slides with a gel electrolyte sandwiched between them

without the electroactive polymer layer, were used as references for the background collection.

Then, the real devices were placed in the sample compartment and leads were attached (the

counter and the reference leads are connected to one another) to control potential and the

absorption (or %transmission) was monitored.

2.3.2 Colorimetry

In situ Colorimetry has also been utilized to precisely map the colors associated with EC

polymers. In situ Colorimetry experiments were done with a CIE recommended normal/normal

(0/0) illuminating/viewing geometry in which the sample was illuminated from behind by a D50

(5000 K) light source in a light booth that eliminates the external light. The experimental setup

used for colorimetric analysis of single polymer films and devices is similar to the one used for

spectroelectrochemistry.

Color coordinates were recorded in CIE 1931 Y xy format. Y (Lv) stands for photometric

luminance with units of cd/m2 and xy are the chromaticity coordinates. Relative luminance,

Y/Yn, is the normalized luminance of the sample relative to the standard (reference white). The

48

relative luminance could be multiplied by 100 to report % relative luminance, (Y/Yn×100).

Relative values are more practical since it is very difficult to reproduce absolute values. We

report data in L*a*b* color coordinates since they are all relative to the standard illuminant and

the reference and this eliminates the experimental errors that might arise from the viewing angle,

aging of light source and etc. Note that L* in L*a*b* color space stands for lightness and it is a

function of luminance (Y).55

The most essential feature of displays, the contrast, is the ratio of the darkest color to the

brightest color. High contrast displays are desired since they are much better perceived by human

eye. Contrast can arise either from luminance or color differences and it is dependent on the

illuminant and the viewing angle. Luminance contrast (Michelson contrast) is calculated as

follows,

𝐶𝐶 = (𝑌𝑌𝑚𝑚𝑐𝑐𝑥𝑥 −𝑌𝑌𝑚𝑚𝑚𝑚𝑛𝑛 )(𝑌𝑌𝑚𝑚𝑐𝑐𝑥𝑥 +𝑌𝑌𝑚𝑚𝑚𝑚𝑛𝑛 )

where Ymax and Ymin are the maximum and minimum luminance of the brightest and the

darkestpoints on the display. The contrast, C, ranges from O to 1. 63, 64

2.3.3 Composite Coloration Efficiency (Tandem Chronocoulometry /Chronoabsorptometry) and Switching Times

Composite coloration efficiency method introduced by our group allows the quantitative

comparison of different systems.21 A tandem chronoabsorptometry/chronocoulometry

experiment was used to calculate composite coloration efficiency and switch time at 95% of the

total change in optical contrast (%T) at λmax. Coloration efficiencies were assessed by

monitoring the %T at a constant wavelength as a function of inserted (injected/ejected) charge.

The wavelength of maximum contrast (λmax) between two extreme redox states was obtained

from spectroelectrochemistry experiments. To obtain polymer and device switching times and

composite coloration efficiency, the Cary 500 spectrophotometer was switched to Kinetics-

49

Transmission mode and average time was set to minimum (0.033 s.). The %T was recorded at

λmax by time (chronoabsorptometry) while the film was brought to two redox extremes by the

applied square potential waveform at intervals of 10 s. The change in charge density versus time

(30 data points/s.) was recorded for further calculations (chronocoulometry). The composite

coloration efficiencies were calculated from the following equations.19, 21, 65

𝜂𝜂 = 𝐶𝐶𝐶𝐶𝑐𝑐𝐶𝐶𝑟𝑟𝑐𝑐𝐶𝐶𝑚𝑚𝐶𝐶𝑛𝑛 𝐸𝐸𝐸𝐸𝐸𝐸𝑚𝑚𝑐𝑐𝑚𝑚𝑐𝑐𝑛𝑛𝑐𝑐𝑦𝑦 = ∆𝑊𝑊𝑂𝑂(𝜆𝜆𝑚𝑚𝑐𝑐𝑥𝑥 )𝑄𝑄𝑑𝑑

where ΔOD is the change in optical density and Qd is the injected/ejected charge per unit

electrode area calculated using the following equations. The ideal material or device would

exhibit a large transmittance change with a small amount of charge.

𝐶𝐶ℎ𝑐𝑐𝑛𝑛𝑎𝑎𝑐𝑐 𝑚𝑚𝑛𝑛 𝐶𝐶𝑝𝑝𝐶𝐶𝑚𝑚𝑐𝑐𝑐𝑐𝑐𝑐 𝑑𝑑𝑐𝑐𝑛𝑛𝑠𝑠𝑚𝑚𝐶𝐶𝑦𝑦 = ∆𝑊𝑊𝑂𝑂 = |𝐴𝐴0.95 − 𝐴𝐴𝑟𝑟𝑐𝑐𝑑𝑑 | = log �%𝑇𝑇0.95

%𝑇𝑇𝑟𝑟𝑐𝑐𝑑𝑑�

Injected ejected charge difference;

𝑄𝑄𝑑𝑑 = 𝑄𝑄0.95 − 𝑄𝑄𝑟𝑟𝑐𝑐𝑑𝑑

Switching time;

𝐶𝐶𝑠𝑠 = 𝐶𝐶0.95 − 𝐶𝐶𝑟𝑟𝑐𝑐𝑑𝑑

2.3.4 Optical Stability of Polymer Films and Devices

The optical stability data of polymer films and ECDs were obtained by monitoring the

lightness (L*) or % T (at the specified wavelength) as a function of time while a square potential

waveform is applied at desired time intervals (5-10 seconds).

2.4 Standard Two-Probe Surface Resistivity Measurement

A conventional two-probe conductivity measurement method, derived from Ohm’s Law,

was employed in this section. The surface resistance (Rs) and thus the surface resistivity (ρs) of

thin PEDOT:PSS film and a thin film of gold (Au) deposited on a non-conducting polyester

membrane (PETE) was measured. The surface resistance (Rs) is the ratio of DC voltage (V)

50

applied to two parallel electrodes to the current flowing between the electrodes (𝑅𝑅𝑠𝑠 = 𝑒𝑒𝐼𝐼� ). The

surface resistivity (ρs) is the ratio of the potential drop per the distance between electrodes to the

current per unit length of electrodes, as shown by the following equation.46, 66, 67

𝜌𝜌𝑠𝑠 =𝑒𝑒𝑐𝑐

𝐼𝐼𝑏𝑏

�

Even though it is common to see the unit of ohms/square, the correct way to report surface

resistivity is in ohms. The setup for the two probe surface resistivity measurement is illustrated

in Figure 2-1. Copper tape contacts were deposited on the films parallel to each other. The

distance between the electrodes was maintained equal to their length, giving a square area. The

sheet resistance between the parallel copper tape electrodes was measured with a multi-meter.

The geometry (lengths) is not important since they cancel each other out in the equation.

Figure 2-1 Setup for a standard two-probe surface resistivity measurement

2.5 Dual Film Technique

Two electrochromic polymer films with different absorption ranges along the visible

spectrum were either electro-deposited or sprayed onto cuvette size ITO-coated glass slides. All

polymer films were conditioned under repeated potential scanning before use. Two ITO

electrodes with polymer films on them were placed back-to-back in a 1 cm quartz cell filled with

Rs

ab

t

51

0.1 M electrolyte solution. The potentials on these two working electrodes (ITO electrodes) were

controlled separately by a bipotentiostat and they were set as working electrodes 1 and 2. A Ag

wire was used as a pseudo-reference electrode and a Pt wire as a counter electrode. Counter and

reference electrode wires were arranged so that they face both working electrodes. In situ color

coordinates, relative luminance values, and electromagnetic spectra in the visible region were

recorded from the dual-polymer system upon application of different potentials to different

working electrodes.

2.6 Electrochromic ECD Construction

General schemes and more detailed explanations for the construction of ECDs will be

given in the following chapters.

Preparation of gel electrolyte: The transparent, highly conductive (2.5 mS/cm) gel

electrolyte used in the devices discussed in this work was composed of a solution of 10 mL PC,

1.1 g PMMA, 0.5 M TBAP and 10 mg glass beads. The gel was stirred and heated on a hot plate

(<60° C) for about four hours until it reached a honey-like consistency.

2.6.1 Window Type Absorption/Transmission Electrochromic Devices (ECDs)

Window type absorption/transmission ECDs were constructed by pairing a cathodically

coloring polymer to non-electrochromic but electroactive transparent polymer. Polymers were

deposited on ITO/glass (1''×1'') electrodes. Electrochromic polymer was oxidatively doped while

the other was neutral prior to device assembly in order to ensure charge balance. Then, these two

films were sandwiched with a gel electrolyte between them. The device was encapsulated by a

paraffin wax that is resistant to propylene carbonate and stopped the immediate leakage of the

gel electrolyte. The wax was melted and hardened immediately after application to the four sides

of the device with cotton swabs. Since in its solid phase the wax is brittle, it was supported by a

52

commercial epoxy that cures in 60 seconds. The same encapsulation method was followed for

all types of devices that will be introduced in this dissertation.

2.6.2 Pseudo-Three-Electrode ECDs

Electrochromic polymer films SprayDOT-Green 145 and SprayDOT-Purple 101 were

sprayed on ITO/glass electrodes (1''×1''). PProDOP-N-EtCN, the non-color changing but

electroactive counter electrode polymer, was electropolymerized on ITO-coated glass slides by

repeated potential scanning from a 0.1 M tetrabutylammonium perchlorate (TBAP)/propylene

carbonate (PC) solution containing 10 mM monomer. All polymer films were electrochemically

conditioned by sweeping the potential. Physically, the construction of the Pseudo 3-Electrode

Device is similar to absorptive/transmissive windows in such that it consists of two

absorptive/transmissive windows in series sharing a counter electrode. The thicknesses of the

PProDOP-N-EtCN films on the counter electrodes were set to ensure charge balance with the

polymer they were facing. Cathodically coloring polymer films were fully oxidized (brought to

their transparent state) and non-color changing films of PProDOP-N-EtCN were fully neutralized

to improve the charge balance prior to the assembly of the device. The SprayDOT-Green 145

and SprayDOT-Purple 101 films were then coated with gel electrolyte and then the PProDOP-N-

EtCN films were closed on top of them. Two devices were connected in series so that the

counter electrode of each device was back-to-back and connected with a copper tape to serve as a

conjunct counter electrode to the whole device. The SprayDOT-Green 145 serves as a working

electrode 1 and the SprayDOT-Purple 101 serves as working electrode 2. The devices were

encapsulated to allow long-term testing.

2.6.3 Three-Electrode ECDs

Three-Electrode electrochromic display device consists of 2 working electrodes and a

counter electrode whose potentials are controlled separately. Working electrodes were prepared

53

as explained above for pseudo-3 electrode ECDs. Counter electrode that is to be sandwiched

between working electrodes 1 and 2 was made of a flexible, porous, transparent and conductive

material. In order to establish this type of electrode (PETE/PEDOT:PSS), highly transmissive

polyester membranes were soaked in PEDOT:PSS/DMSO formulation and they were cured for 2

hours in a vacuum oven at 120 oC to reach a highly conductive state. Highly soluble, non-

electrochromic, electroactive, transparent polymer, PTMA, was blended with PMMA in order to

prevent its dissolution in to the gel electrolyte. PTMA and PMMA were dissolved in toluene in a

weight ratio of 1:4 (w/w) and the final concentration of solution was 1.25 mg polymer/mL of

solvent. The solution was sprayed on to the new type of electrode PETE/PEDOT:PSS to be

utilized as a counter electrode (PETE/PEDOT:PSS/PTMA). Electrode layers were separated by a

gel electrolyte which utilizes the charge transport. The device was encapsulated.

2.6.4 RGB Color Space Five-Electrode ECDs

RGB Color Space 5-electrode electrochromic display device consists of 3 working

electrodes and two counter electrodes whose potentials are controlled separately. ITO/glass was

chosen as an electrode material for the outermost electrodes, working electrode 1 and working

electrode 3, in order to establish a strong foundation for the multi-layered device. Inner

electrodes were made of (PETE/PEDOT:PSS/PTMA) of whose preparation was described above

for 3-electrode ECDs. Electrochromic polymers which are red and green at their neutral states

were sprayed on to ITO/glass electrodes. Electrochromic polymer which is blue at its neutral

state was sprayed on PETE/PEDOT:PSS and named working electrode 2. PTMA:PMMA, non-

electrochromic, electroactive, transparent polymer blend, was sprayed on PETE/PEDOT:PSS to

be utilized as counter electrodes 1 and 2. Counter electrodes 1 and 2 were sandwiched in

between the working electrodes 1, 2 and 3 and maintained the charge balance in the device.

54

Electrode layers were separated by a gel electrolyte which utilizes the charge transport. The

device was encapsulated.

55

CHAPTER 3 DUAL-POLYMER ELECTROCHROMIC FILM CHARACTERIZATION USING

BIPOTENTIOSTATIC CONTROL

Conjugated conducting polymers receive particular attention for their potential use in

controllable light-reflective or light-transmissive electrochromic displays for optical information

and storage.13 They have fast switching speeds, high contrast ratios and high coloration

efficiencies. Several are available as solution-processable materials, and their electrochromic

properties and color states can be synthetically tuned.19

ECDs are designed to modulate absorbed, transmitted, or reflected incident

electromagnetic radiation, through the application of an electric field across the electrochromic

materials within the device.13 Often, an ECD includes two electrochromic materials that have

complementary optical (cathodic and anodic coloring) properties allowing both electrochromes

to contribute to the optical response of the device. The absorptive/transmissive-type ECD

operates with a reversible switching of the electrochrome between a colored state and a bleached

state. Both working and counter electrode are transparent so that light can pass through the

device. Reflective ECDs have also been developed where the active electrochromic polymer is

deposited onto an outward-facing reflective electrode, such as gold deposited onto a flexible, ion

permeable substrate.29

Although choice of electrochromic conjugated conducting polymers can provide colors

across the entire range of the visible spectrum, colors achieved through color addition, especially

in intermediate oxidation states is not always obvious in dual polymer additive devices. Here,

we introduce a novel analytical method, which allows the systematic variation of color states of

pairs of electrochromic conjugated conducting polymers with simultaneous

spectroelectrochemical and colorimetric characterization of the resulting color summation. In

this method, the polymers are prepared as films on ITO/glass substrates and mounted back-to-

56

back in transmission mode in a spectroelectrochemical cell. A bipotentiostat provides separate

electrochemical control of individual polymer color states. By the dual-polymer electrochromic

technique through bipotentiostatic control, we demonstrate the generation of new color states by

coupling existing polymers. Our choices of polymers for this study, with examples prepared

both by monomer electropolymerization and the spray-coating of soluble polymers,26 have been

from the available PXDOT and PXDOP families, although, in principle, this approach is

applicable to numerous electrochromes.

3.1 Color Mixing

In its simplest sense, color mixing allows the matching of complex colors through the

combination or formulation of color components.49, 56 In the dual-polymer electrochromic film

characterization technique reported here, the results can be used to design dual polymer ECDs

where the color summation of the films in any of their color states can be accessed as the films

oxidation states are controlled independently. The key to this is understanding the coloration

process during the doping /dedoping of the polymer films. These “primary” colors can be

summed to more complex colors by passing white light simultaneously through two films and

observing the transmitted light. Color engineering for ECDs will let us foresee colors that are

accessible after the primary components are matched logically. Each dual system will have a

palette of colors available made possible by independently varying the oxidation state of the two

films.

According to color mixing theory, it is possible to obtain all colors required once additive

or subtractive primary colors are available.38, 41 The theory also states that, when colors are

mixed, the chromaticiy coordinates of the resulting color lies on the line joining the two original

chromaticity coordinates. Thus, one can determine the approximate hue of the device that

constitutes several layers of electrochromic polymers. Other than structural modifications, a fine

57

tuning of color is also possible by adjusting doping levels and the film thickness in

electrochromic devices. The ideal electrochromic device should exhibit high coloration

efficiency, meaning that very high optical contrast by introduction of least amount of charge,

high stability and fast switch times.19 Until now, multicolor displays have been shown to be

possible by combining two ECDs in series and controlling them independently or by patterning

the electrode surfaces.19, 41, 68, 69

3.2 Choosing A System for Dual-Polymer Technique by Fundamental Properties (EDOT, ProDOP and ProDOT-Hx2)

In this work we have used PEDOT, PProDOP and dihexyl substituted poly(3,4-

propylenedioxythiophene) (PProDOT-Hx2) as exemplary EC polymers whose colors can be

summed to provide new colors not possible with the single films alone.26 The repeat unit

structures of the polymers, along with photographs of the polymer films in their oxidized and

reduced states are shown in Figure 3-1. Each of these polymers is cathodically coloring with

orange, blue, and purple/magenta colors in their fully neutral states.

Also shown in Figure 3-1 is a schematic of the cell used for the dual-polymer EC

characterization method. In this construction, EC polymer films are deposited onto separate

ITO/glass slides and placed back-to-back in a cuvette in order to mimic the combined light

absorption properties of a dual polymer ECD. The redox state of each film (WE 1 and WE 2) is

then controlled independently with a bipotentiostat.

Understanding the coloration process during the oxidation and charge neutralization of the

polymer films is of paramount importance in establishing the new color palettes. To understand

this fully, it is necessary to obtain the electrochemical and optical properties of the separate

polymer films. Electrochemical characterizations on Pt-button electrodes and

58

spectroelectrochemical experiments on ITO-coated glass electrodes were used to establish

baseline properties.

Figure 3-1 Chemical structures of the polymers that are used in dual-polymer electrochromic method and the photographs of their neutral (N) and doped (D) states on ITO/glass electrode with the schematic of the dual-polymer electrochromic characterization cell.

3.2.1 Film Deposition

In order to obtain thin films of the EC polymers, both EDOT and ProDOP were

electrochemically polymerized onto Pt-button electrodes from a 0.1 M LiClO4/PC solution

containing 10 mM monomer by repeated scanning as shown in Figure 3-2 and Figure 3-3,

respectively. During the first anodic scans, a single peak was observed corresponding to

irreversible oxidation of the monomers indicating formation of radical cations (Figure 3-2 inset).

The peaks of monomer oxidation are observed at +1.02 V for EDOT and +0.56 for ProDOP vs.

Fc/Fc+. Subsequent scanning shows evolution of a redox response at lower potentials attributed

the polymer oxidation and charge neutralization. PProDOT-Hx2 was drop-cast from a 5mg/mL

polymer/toluene solution onto a Pt-button electrode after being filtered through 0.45 μm PTFE

filter.

CERE

WE 2 WE 1

CERE

WE 2 WE 1

59

Figure 3-2 The repeated potential scanning electropolymerization of EDOT from 10 mM monomer in 0.1 M LiClO4/PC solution on a Pt-button electrode at a scan rate of 20 mV/s.

Figure 3-3 The repeated potential scanning electropolymerization of ProDOP from 10 mM monomer in 0.1 M LiClO4/PC solution on a Pt-button electrode at a scan rate of 20 mV/s.

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5-6

-4

-2

0

2

4

6

8

10

12

Cur

rent

Den

sity

(mA

/cm

2 )

Potential (V) vs. Fc/Fc+

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5-1

0

1

2

3

4

5

Cur

rent

Den

sity

(mA

/cm

2 )

Potential (V) vs. Fc/Fc+

-1.5 -1.0 -0.5 0.0 0.5 1.0-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Cur

rent

Den

sity

(mA

/cm

2 )

Potential (V) vs Fc/Fc+

60

3.2.2 Polymer CV and Scan Rate Dependence

After deposition, all films were rinsed with monomer-free electrolyte solutions and cyclic

voltammograms (CVs) were recorded at scan rates ranging from 20 to 300 mV/s as shown in

Figures 3-4 to 3-6. A linear increase of the current with scan rate is observed for each film,

indicative of a surface adhered electroactive polymer film. An important aspect observed in

these experiments is that the current passing through the PEDOT film is an order of magnitude

greater than the current passing through the PProDOP (prepared with the same number of

deposition scans) and PProDOT-Hx2 films. This demonstrates the highly effective

electropolymerization and switching characteristics of PEDOT. This shows that during the

preparation of dual film devices, it is important to balance the overall amount of electroactive

polymer on each electrode, usually using film thickness as the operational parameter, in order to

obtain a balanced optical response.

Figure 3-4 Cyclic voltammograms of PEDOT in 0.1 M LiClO4/PC at scan rates of (a) 20, (b) 50, (c) 100, (d) 150, (e) 200, and (f)300 mV/s. Film was electrochemically polymerized onto the Pt-button electrode from a 0.1 M LiClO4/PC solution containing 10 mM monomer by repeated scanning.

-2.0 -1.5 -1.0 -0.5 0.0 0.5

-30

-20

-10

0

10

20

30

40

f

a

Curre

nt D

ensi

ty (m

A/cm

2 )

Potential (V) vs Fc/Fc+

a

f

61

Figure 3-5 Cyclic voltammograms of PProDOP in 0.1 M LiClO4/PC at scan rates of (a) 20, (b) 50, (c) 100, (d) 150, and (e) 200 mV/s. Film was electrochemically polymerized onto the Pt-button electrode from a 0.1 M LiClO4/PC solution containing 10 mM monomer by repeated scanning.

Figure 3-6 Cyclic voltammograms of PProDOT-Hx2 in 0.1 M LiClO4/PC at scan rates of (a) 20, (b) 50, (c) 100, (d) 150, (e) 200, and (f) 300 mV/s. Film was prepared by drop-casting onto a Pt-button electrode from 5 mg/mL polymer/toluene solution.

3.2.3 Spectroelectrochemistry

Using the above CV experiments as a means of determining the correct potential ranges for

switching and evaluating the stability of the electroactivity of the polymer films,

-1.2 -1.0 -0.8 -0.6 -0.4-4-3-2-10123456

e

a

Curr

ent D

ensi

ty (m

A/cm

2 )

Potential (V) vs Fc/Fc+

a

e

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

f

a

Curr

ent D

ensi

ty (m

A/cm

2 )

Potential (V) vs Fc/Fc+

a

f

62

spectroelectrochemical and colorimetric experiments were conducted to elucidate the separate

optical characteristics of the polymer films alone. Films were deposited on ITO coated glass

slides potentiostatically for PEDOT (1.6 V for 20 s), galvanostatically for PProDOP (0.11 mA

for 1000 s) and by spray casting (5 mg/mL from toluene) for PProDOT-Hx2. The film

thicknesses were adjusted to between 100-300 nm so that the absorbances of the polymer films at

λmax in their neutral states were equal. The spectroelectrochemical series for each of the polymer

films are shown in Figures 3-7 to 3-9. In their neutral states, PEDOT appears deep blue

(absorbing between 1.6 and 2.75 eV), PProDOP appears orange (absorbing between 2.2 and 3.0

eV) and, PProDOT-Hx2 appears purple (absorbing between 1.8 and 3.0 eV). As these polymer

films are doped, charge carrier states emerge with the majority of the light absorption for each

polymer being in the NIR which results in highly transmissive films. Note, in the

spectroelectrochemical series for PProDOP, we step to +0.5 V which is substantially higher than

the potential window used in the CV. This higher applied potential allowed us to fully oxidize

the film and attain the most transmissive form. This ability to form a transmissive state for each

of these cathodically coloring polymers is important when considering them for use in EC

displays and absorption/transmission windows. Converting to transmittance, we note that Δ%T

values at λmax (632 nm, 522 nm and 571 nm) are 55%, 73% and 65% for PEDOT, PProDOP and

PProDOT-Hx2, respectively.

63

Figure 3-7 Spectroelectrochemistry of potentiostatically deposited, redox switched, PEDOT film at applied potentials of (a) -1.45 to (s) +0.35 V vs Fc/Fc+ in increments of 0.1 V in 0.1 M LiCLO4/PC solution.

Figure 3-8 Spectroelectrochemistry of galvanostatically deposited, redox switched, PProDOP film at applied potentials of (a) -1.7 to (s) +0.1 V vs Fc/Fc+ in increments of 0.1 V in 0.1 M LiCLO4/PC solution.

400 600 800 1000 1200 1400 16000.0

0.5

1.0

1.5

s

a

Abs

orba

nce

(a.u

.)

wavelength (nm)

a

s

400 600 800 1000 1200 1400 16000.0

0.5

1.0

1.5

2.0

s

a

Abs

orba

nce

(a.u

.)

wavelength (nm)

a

s

64

Figure 3-9 Spectroelectrochemistry of spray-cast, redox-switched, PProDOT-Hx2 film at applied potentials of (a) -0.67 to (m) +0.53 V vs Fc/Fc+ in increments of 0.1V, in 0.1 M LiCLO4/PC solution.

3.2.4 Tandem Chronocoulometry and Chronoabsorptometry

Tandem chronoabsorptometry and chronocoulometry experiments allow calculation of

composite coloration efficiency (CE). By choosing 95% of the optical density change the

transmittance of the reduced films is compared to that the of oxidized films. The amount of time

to reach 95% of the full optical density is chosen as nearly all of the optical change has occurred

and a direct comparison of polymers that switch at different rates can be made.21 Polymer films

with similiar switching times and contrast ratios were chosen for application to the dual-film

technique. The coloration efficiency for PEDOT is 280 cm2/C and the 95% switch time is 1.7 s

with a charge density of 3.4 mC/cm2, while for PProDOP the coloration efficiency is 224 cm2/C

and the 95 % switch time is 0.9 s with a charge density of 1.6 mC/cm2, and for PProDOT-Hx2

the coloration efficiency is 519 cm2/C and the 95% switch time is 0.6 s with a charge density of

1.4 mC/cm2 (Figures 3-10 to 3-12). All three polymer films have substantial EC switching

occuring in the sub-second time frame and are fully switched within two seconds at most.

400 600 800 1000 1200 1400 16000.0

0.2

0.4

0.6

0.8

1.0

mm

a

Abs

orba

nce

(a.u

.)

wavelength (nm)

a

65

Figure 3-10 Tandem chronoabsorptometry and chronocoulometry experiments for PEDOT in 0.1 M LiClO4/PC solution. (-1.45 to +0.55 V vs. Fc/Fc+, held for 10 s at each potential at 632 nm)

Figure 3-11 Tandem chronoabsorptometry and chronocoulometry experiments for PProDOP (-1.7 to +0.1 V vs Fc/Fc+, held for 10 s at each potential at 522 nm) in 0.1 M LiClO4/PC solution.

0 2 4 6 8 10 12 14 16 18 200

1

2

3

4

5

6

time (sec.)

Cha

rge

Den

sity

(mC

/cm

2 )

10

20

30

40

50

60

% Transm

ittance

0 2 4 6 8 10 12 14 16 18 20

0.0

0.5

1.0

1.5

2.0

2.5

3.0

time (sec.)

Cha

rge

Den

sity

(mC

/cm

2 )

25

30

35

40

45

50

55

60

65

% Transm

ittance

66

Figure 3-12 Tandem chronoabsorptometry and chronocoulometry experiments for PProDOT-Hx2 in 0.1 M LiClO4/PC solution. (-0.67 to +0.53 V vs. Fc/Fc+, held for 10 s at each potential at 571 nm)

3.2.5 Colorimetry

Since color is subject to the response, sensitivity, and perception of the human eye,

elaboration on EC properties are best accomplished with an accurate quantitative measure of the

color. In situ color coordinates (hue and saturation) and relative luminance (the amount of light

transmitted through the polymer film) values were recorded for each polymer film separately

(Figures 3-13 to 3-15). Considering the three polymer films in their fully reduced forms,

PEDOT has a and b values of -5 and -37, respectively, giving it a dark blue color with a relative

luminance of 32%, PProDOP has a* and b* values of 31 and 75, respectively, giving it an orange

color with a relative luminance of 50%, and PProDOT-Hx2 has a* and b* values of 14 and -45

giving it a purple color with a relative luminance of 24%. Note, there is variability in the

measured L*, a*, and b* values measured as a function of subtle changes in film thickness and

applied potential. For example, in earlier work we measured neutral PEDOT as L* = 20, a* =

15, and b* = -43.70 While in both instances the PEDOT films are obviously a deep blue, this

0 2 4 6 8 10 12 14 16 18 20

0.0

0.5

1.0

1.5

2.0

2.5

time (sec.)

Cha

rge

Den

sity

(mC

/cm

2 )

10

20

30

40

50

60

70

80

%Transm

ittance

67

points out the sensitivity of the method. Also note that the a* and b* values reported for PEDOT

in this earlier work are close to the values reported here for PProDOT-Hx2, even they have

different colors. The color difference in this case arises from different L* values they have.

When the films are completely oxidized they all are converted into highly transmissive and

sky-blue colored films. Now, PEDOT exhibits a* and b* values of -2 and -4, respectively, with

a relative luminance of 82%, PProDOP exhibits a* and b* values of -3 and -6, respectively, with

a relative luminance of 57%, and PProDOT-Hx2 exhibits a* and b* values of -2 and -4,

respectively, with a relative luminance of 86%, demonstrating the cathodic coloration properties

of each of these polymers.

As shown by the luminance change in Figure 3-15, PProDOT-Hx2 possesses the highest

contrast ratio in the visible region of the three polymers studied, having Δ%Y of 62% while

PEDOT and PProDOP have 50% and 10-20% (from fully neutral to the oxidized form),

respectively. The reduced contrast of PEDOT relative to PProDOT-Hx2 is due to the strong NIR

absorption that is found in the oxidized form of PEDOT providing a visible light absorption tail

in the red region of the spectrum, which is lower in intensity for the substituted PProDOTs. The

simple loss of absorption in the visible region for the thiophene derivatives upon oxidation is

seen to be more complicated in PProDOP. As the polymer has a higher band gap, the neutral

form is more transmissive to visible light than either of the thiophene derivatives. The initial

formation of a polaron during oxidation gives an absorption in the visible region, which results in

an initial loss of luminance (Figure 3-14). This polaron absorption is subsequently bleached

upon full oxidation and a highly transmissive (%Y ~ 60%), light gray state is ultimately reached.

These luminance changes with applied potential will play a strong role in the EC response of the

dual film systems to be described below.

68

Figure 3-13 Relative luminance as a function of applied potential of PEDOT in 0.1 M LiClO4 solution

Figure 3-14 Relative luminance as a function of applied potential of PProDOP in 0.1 M LiClO4 solution

-1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2

30

40

50

60

70

80

90

Rel

ativ

e Lu

min

ance

(%Y)

Potential (V) vs. Fc/Fc+

-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4

25

30

35

40

45

50

55

60

65

Rel

ativ

e Lu

min

ance

(%Y)

Potential (V) vs. Fc/Fc+

69

Figure 3-15 Relative luminance as a function of applied potential of PProDOT-Hx2 in 0.1 M LiClO4 solution

3.3 PProDOP/PEDOT and PProDOP/PProDOT-Hx2 Dual Systems

Two ITO working electrodes with different EC polymer films on them were prepared in a

similar manner to the films that were used for spectroceletrochemical and colorimetric

characterizations. Both ITO electrodes with two different polymer films on them and under

separate potentiostatic control were placed back to back in a 1cm quartz cell, with a Ag wire as a

reference electrode and a Pt wire as a counter electrode. ITO-coated glass slides were used as

the working electrodes (Figure 3-1). In situ color coordinates and electromagnetic spectra in the

visible region were recorded from the dual-polymer system upon application of different

potentials to different working electrodes in a 0.1 M LiClO4/PC solution.

Absorbance spectra of PEDOT and PProDOP films, were taken separately at their reduced

(both at -1.35 V vs. Fc/Fc+) and oxidized (PEDOT at +0.45 V and PProDOP at -0.05 V vs.

Fc/Fc+) states and these spectra were summed theoretically in order to attain a perspective of the

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

20

30

40

50

60

70

80

90

Rel

ativ

e Lu

min

ance

(%Y)

Potential (V) vs. Fc/Fc+

70

combination of the optical response expected from the dual-film EC method. This is

demonstrated by the single film red and black curves in Figure 3-16, along with the theoretical

summation represented by the green curve (Note the green curve in Figure 3-16 (b) is difficult to

see behind the blue curve). The blue curves in Figure 3-16 show the direct spectral response

from the combined films. The theoretical and experimental spectra are shown to overlay one

another completely for the oxidized films, and are quite similar for the reduced films. The color

transmitted through the stacked films thus appears completely different from the component

films and is difficult to foresee. The experimental summation spectrum proves that the dual

system enables the physical addition of the optical properties of two different polymer systems

thus motivating the perception of new colors which are the mixtures of each polymer.

Two dual-film systems were studied colorimetrically from the three polymer films

employed. The color palettes in Figures 3-17 and 3-18 show the photographs and the L*a*b*

color coordinates as a function of the separate potential applied to each film. These color

palettes can be used to tune in the accessible colors from a dual-film EC device. For example, as

shown in Figure 3-17 as PProDOP is reduced and held in its orange state while sequentially

oxidizing PEDOT, the luminance of the dual-film system increases from 59 to 68 as PEDOT is

converted from a dark blue to a transmissive film. Along this track, the orange color dominates

as the film retains an orange/brown hue. Holding PEDOT in the deep blue neutral state while

sequentially oxidizing the PProDOP yields a more distinct visual response with the

brown/orange film changing to a green-tinted gray. Full oxidation of both films gives the

lightest gray state.

71

Figure 3-16 UV–vis-NIR spectra of PProDOP/PEDOT from dual-polymer electrochromic setup at (a) reduced, and (b) oxidized states in 0.1 M LiClO4/PC solution

400 600 800 1000 1200 1400 16000.0

0.5

1.0

1.5

2.0

2.5 PProDOP PEDOT theoretical summation of

PProDOP and PEDOT experimantal summation of

PProDOP and PEDOT

Abs

orba

nce

(a.u

.)

wavelength (nm)

a

400 600 800 1000 1200 1400 16000.0

0.5

1.0

1.5

2.0

2.5 PProDOP PEDOT theoretical summation of

PProDOP and PEDOT experimantal summation of

PProDOP and PEDOT

Abs

orba

nce

(a.u

.)

wavelength (nm)

b

72

Figure 3-17 L*a*b* color coordinates and photography for PProDOP/PEDOT in 0.1 M LiClO4/PC

The color changes are even more distinct in the PProDOP/ PProDOT-Hx2 couple seen in

Figure 3-18. When the PProDOP film is held reduced while oxidizing the PProDOT-Hx2 film

the luminance value increases from 56 to 72 due to the PProDOT-Hx2 converting from dark

purple to transmissive blue and resulting in a distinct reddish/purple to orange color change in

the dual-film system. As the PProDOP is oxidized and the PProDOT-Hx2 held reduced, the

conversion to the blue state is observed as the PProDOP becomes highly transmissive. Finally

the highest luminance is observed as both films are oxidized.

-1.35 V

vs. Fc/Fc+

-1.20 V -1.05 V -0.90 V -0.75 V -0.60 V -0.45 V

L*=64a*= -5b*=-37

L*=63a*= -6b*=-38

L*=64a*= -6b*=-37

L*=64a*= -6b*=-37

L*=65a*= -6b*=-36

L*=70a*= -4b*=-28

L*=79a*= -2b*=-17

-1.35 V L*= 76a*=31b*=75

L*=59a*=25b*=50

L*=56a*=25b*=43

L*=58a*=25b*=48

L*=56a*=26b*=44

L*=59a*=26b*=50

L*=58a*=28b=*47

L*=68a*=30b*=64

-1.20 V L*= 76a*=32b*=74

L*=53a*=25b*=36

L*=51a*=24b*=27

L*=53a*=25b*=35

L*=51a*=24b*=27

L*=53a*=26b*=35

L*=52a*=24b*=27

L*=59a*=29b*=43

-1.05 V L*= 75a*=31b*=72

L*=58a*=24b*=43

L*=55a*=24b*=39

L*=50a*=18b*=14

L*=55a*=24b*=39

L*=59a*=25b*=47

L*=57a*=26b*=43

L*=67a*=28b*=59

-0.90 V L*= 68a*=25b*=50

L*=51a*=19b*=18

L*=50a*=18b*=15

L*=50a*=19b*=18

L*=50a*=19b*=16

L*=51a*=19b*=18

L*=52a*=18b*=14

L*=57a*=21b*=26

-0.75 V L*= 67a*= 11b*= 8

L*=56a*= 2b*=-17

L*=53a*= 6b*=-11

L*=59a*= 2b*=-15

L*=52a*= 8b*=-8

L*=-56a*= 6b*=-11

L*=-55a*= 10b*= -2

L*=64a*= 5b*= -1

-0.60 V L*= 73a*= 2b*= -5

L*=57a*= 2b*=-17

L*=56a*= 2b*=-15

L*=57a*= 1b*=-17

L*=57a*= 1b*=-16

L*=58a*= 1b*=-16

L*=59a*= 3b*=-11

L*=64a*= 2b*= -8

-0.45 V L*= 78a*= -1b*= -5

L*=60a*= 0b*=-18

L*=61a*= 0b*=-18

L*=61a*= 0b*=-18

L*=62a*= -1b*=-18

L*=62a*= 0b*=-17

L*=64a*= 0b*=-15

L*=67a*= 1b*=-11

PProDOP

PEDOT

73

Figure 3-18 L*a*b color coordinates and photography for PProDOP/PProDOT-Hx2 in 0.1 M LiClO4/PC

3.4 Conclusions

This work demonstrates a new dual-polymer electrochromic film characterization

technique allowing electrochromic polymer film couples to be studied and new colors generated.

The color observed and thus the color coordinates read from the dual-polymer set-up for the

coupled polymer films were different than the data collected from these polymers when they

were studied separately. For example; coupling PEDOT and PProDOP films in their neutral

states resulted in a new color red/brown (L*=59 a*=25 b*=50) which is totally different than the

original colors these polymers show at their neutral states, dark blue (L=64* a*=-5 b*=-38) and

-0.85 V -0.65 V -0.45 V -0.25 V -0.05 V 0.15 V

L*= 56a*= 14b*=-45

L*= 56a*= 14b*=-45

L*= 56a*= 14b*=-45

L*= 59a*= 12b*=-40

L*= 81a*= 8b*=-11

L*= 92a*= -1b*= -3

-1.35 V L*= 76a*= 31b*= 75

L*= 56a*= 21b*=16

L*= 54a*= 23b*= 16

L*= 54a*= 23b*= 15

L*= 55a*= 22b*= 14

L*= 66a*= 27b*= 33

L*= 72a*= 24b*= 39

-1.20 V L*= 76a*= 32b*= 74

L*= 55a*= 21b*=15

L*= 55a*= 22b*= 16

L*= 55a*= 22b*=15

L*= 54a*= 24b*=16

L*= 67a*= 26b*= 35

L*= 72a*= 23b*= 38

-1.05 V L*= 75a*= 31b*= 72

L*= 55a*= 20b*=13

L*= 53a*= 22b*= 15

L*= 52a*= 22b*= 13

L*= 55a*= 22b*= 14

L*= 68a*= 25b*= 32

L*= 72a*= 22b*= 37

-0.90 V L*= 68a*= 25b*= 50

L*= 52a*= 16b*=-2

L*= 52a*= 18b*= 1

L*= 52a*= 17b*= 1

L*= 53a*= 18b*= 3

L*= 65a*= 20b*= 21

L*= 70a*= 17b*= 26

-0.75 V L*= 67a*= 11b*= 8

L*= 54a*= 9b*=-21

L*= 53a*= 11b*=-18

L*= 54a*= 10b*=-17

L*= 55a*= 11b*=-14

L*= 68a*= 9b*= 2

L*= 73a*= 4b*= 2

-0.60 V L*= 73a*= 2b*= -5

L*= 57a*= 7b*=-28

L*= 56a*= 7b*=-27

L*= 57a*= 7b*=-25

L*= 59a*= 7b*=-22

L*= 72a*= 5b*= -6

L*= 76a*= 0b*= -3

-0.45 V L*= 78a*= -1b*= -5

L*= 60a*= 7b*=-28

L*= 58a*= 8b*=-28

L*= 59a*= 7b*=-27

L*= 60a*= 8b*=-24

L*= 74a*= 4b*= -7

L*= 79a*= -1b*= -4

PProDOP

PProDOT-Hx2

74

orange (L*=76 a*=31 b*=75), respectively. A full palette of colors is accessible by the logical

coupling of electrochromic polymers in the dual-polymer electrochromic set-up and this method

eliminates the need to synthesize new families of polymers to generate different colors. With the

correct choice of materials, the dual system promises many intermediate colors which will enable

the formation of a wide color scale for ECDs.

75

CHAPTER 4 APPLICATION OF BIPOTENTIOSTATIC CONTROL IN A 3-ELECTRODE

ELECTROCHROMIC DEVICE: TOWARDS BLACK TO TRANSMISSIVE AND MULTI-COLORED SWITCHING

The focus of endeavoring synthesis of processable, high contrast EC polymers is to

establish full color palettes for use in displays and printing. Printing technologies and electronic

information transformation displays demand high contrast devices that could be perceived by a

human eye with minimum blurring. This need brings out the requirement for a material

switching in between two extreme optical states, from opaque black to transmissive. There are

inorganic displays switching from black to clear available, yet they are brittle.11 Synthesis of

black to clear switching and processable conjugated polymers had been a challenge for synthetic

chemists until, recently, the work reported and disclosed by Beaujuge et al.71 Other than

synthetic means, new colors can be achieved by physical methods. Researcher have utilized dual

absorptive/ transmissive ECDs which are connected in series for color mixing.36 Since, these

devices constitute many EC layers one can mix colors and determine the approximate hue of the

device. Unfortunately, the use of these devices is limited to electrochromic materials with

complementary optical properties, one anodically coloring and the other cathodically coloring.30,

47 Other than that, as reported by Otero and Padilla, the contrast achieved in dual electrochromic

systems are limited when compared to the contrast from single films. Higher contrast of dual

electrochromic windows is possible only through the careful design and use of an electroactive

yet non-electrochromic highly transmissive polymers as a counter electrode materials.48

We utilized the dual-polymer electrochromic film characterization method in this chapter

in establishing a new color profile including black to transmissive by mixing green and purple.

Here, we introduced the first two black to transmissive switching conjugated polymer ECDs, the

Pseudo 3-Electrode Electrochromic Device (P3-ECD) and 3-Electrode Electrochromic Device

76

(3-ECD). These devices add colors by transmitting light through two working electrodes (coated

with two different electrochromic polymer films) and counter electrodes (coated with non-

electrochromic yet electroactive polymer films) stacked together with a gel electrolyte between

the layers. The electrodes are all under separate potentiostatic control, which enables the multi-

color switching. In this work, we utilized non-color changing, highly transmissive, electroactive

polymers, PProDOP-N-EtCN and PTMA, as counter electrode materials. The P3-ECD and 3-

ECD stand out with their optical contrasts comparable to single films. By utilizing P3-ECD and

3-ECD, other than black and white scripts colored images can be displayed on information

displays. P3-ECD and 3-ECD could also serve as reflective type of devices (i.e. e-book) by use

of a diffuse back-plane such as white paint or a cellulose based support.

4.1 Towards Black to Clear Switching ECDs -Fundamental Properties (SprayDOTTM-Purple 101, SprayDOTTM-Green 145, PProDOP-N-EtCN)

In this work di-ester substituted poly(3,4-propylenedioxythiophene) (PProDOT-

(CH2COOC12H25)2), SprayDOTTM-Purple 101 and SprayDOTTM-Green 145 were chosen as

primary colors. The SprayDOTTM-Green 145, P(EDOT2(ProDOT-(CH2O(2-EtHx))2)2BTD), was

synthesized by alternative addition of electron rich EDOT and 2-ethylhexyloxy-substituted 3,4-

propylenedioxythiophene ProDOT-(CH2O(2-EtHx))2 onto the strong acceptor 2,1,3-

benzothiadiazole (BTD). These primaries can be summed to provide new colors, especially

black which has not been available by synthetic means until the recent work that was published

by Beaujuge et al.71 SprayDOTTM-Purple 101 and SprayDOTTM-Green 145 are cathodically

coloring polymers, SprayDOTTM-Purple 101 switching from a deep magenta (L*= 41 a*= 22

b*= -48) to a highly transmissive gray/blue (L*= 87 a*= -2 b*= -7) and SprayDOTTM-Green 145

switching from a deep green (L*= 60 a*= -23 b*= 12) to transmissive sky blue (L*= 84 a*= -4

77

b*= -6). The repeat unit structures of the polymers, along with photographs of the polymer films

in their oxidized and reduced states are shown in Figure 4-1.

Figure 4-1 Chemical structures of the polymers that are used in dual-polymer electrochromic method and in ECDs, the photographs of their neutral (N) and doped (D) states on ITO/glass electrodes.

Understanding the electrical response and coloration process of the polymer films is

paramount in color and device engineering. Hence, it is essential to obtain the electrochemical

and optical properties of the separate polymer films. Electrochemical characterizations on Pt-

button electrodes, spectroelectrochemical and colorometric experiments on ITO-coated glass

electrodes were used to set optimum conditions and foresee the outcome from the devices.

4.1.1 Film Deposition

In order to obtain thin films of the EC polymers for electrochemical and optical studies,

SprayDOTTM-Purple 101 and SprayDOTTM-Green 145 were drop-cast (on Pt-button electrodes)

or spray-cast (on ITO coated glass electrodes) from 2 mg/mL polymer/toluene solutions after

being filtered through 0.45 μm PTFE filters. SprayDOTTM-Purple 101 and SprayDOTTM-Green

145 films sprayed on ITO/glass were dried under vacuum overnight.

DN

Ox

Red

N D

Ox

Red

NNS

S

O O

S

O O

O O

R R

S

O O

S

O O

O O

R R

n

R = 2-E th ylH ex ylS

O O

RR

R = D o d e c y l

O

O

O

O

SprayDOT - Purple 101 SprayDOT - Green 145 PProDOP-N-EtCN

N D

Ox

Red

PProDOT-(CH2COOC12H25)2 P(EDOT2(ProDOT-(CH2O(2-EtHx))2)2BTD)

78

PProDOP-N-EtCN was electrodeposited on the same set of electrodes by potential

scanning. Cyclic voltammograms for electrodeposition of PProDOP-N-EtCN on a Pt-button

electrode are shown in Figure 4-2 (20 cycles, 50 mV/s, -0.3 V to +0.6 V vs. Fc/Fc+). During the

first anodic scan on Pt-button electrode, a single peak was observed corresponding to irreversible

oxidation of the monomers indicating formation of radical cations. The peak of monomer

oxidation was observed at +0.4 V vs Fc/Fc+. Subsequent scanning shows evolution of a redox

response at lower potentials attributed to the polymer oxidation and charge neutralization.

PProDOP-N-EtCN films did not adhere strongly to ITO/glass, probably due to the N-substitution

which enhances solubility. In order to stabilize the polymer on the transparent electrode, films

were heated at 55 oC under vacuum for 30 minutes. This process might be initiating a cross-

linking between –CN groups and decreasing the solubility of the polymer.

Figure 4-2 Repeated potential scanning electropolymerization of ProDOP-N-EtCN from a 10 mM monomer in 0.1 M TBAP/PC solution on a Pt-button electrode (20 cycles, 50 mV/s, -0.3 V to +0.6 V vs. Fc/Fc+).

-0.2 0.0 0.2 0.4 0.6-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Cur

rent

Den

sity

(mA

/cm

2 )

Potential (V vs. Fc/Fc+)

-0.2 0.0 0.2 0.4 0.6-0.20.00.20.40.60.81.0

(mA

/cm

2 )

V vs. Fc/Fc+

79

4.1.2 Polymer CV and Scan Rate Dependence

CVs were recorded at scan rates ranging from 25 to 300 mV/s as shown in Figures 4-3 to

4-5. A linear increase of the current with scan rate is observed for each film, indicative of a

surface adhered electroactive polymer film.

Figure 4-3 Cyclic voltammograms of PProDOP-N-EtCN in 0.1 M TBAP/PC at scan rates of (a) 25, (b) 50, (c) 75, (d) 100, (e) 150, (f) 200 and (g) 300 mV/s, on a Pt-button electrode. Film was prepared by potential scanning from a 10 mM monomer in 0.1 M TBAP/PC solution (20 cycles, 50 mV/s, -0.3 V to +0.6 V vs. Fc/Fc+).

Figure 4-4 Cyclic voltammograms of the SprayDOTTM-Purple 101 in 0.1 M TBAP/PC at scan rates of (a) 25, (b) 50, (c) 75, (d) 100, (e) 150, (f) 200 and (g) 300 mV/s, on a Pt-button electrode. Film was prepared by drop-casting onto a Pt-button electrode from a 2 mg/mL polymer/toluene solution.

-0.3 -0.2 -0.1 0.0 0.1 0.2

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

g

a

g

Cur

rent

Den

sity

(mA

/cm

2 )

Potential (V vs. Fc/Fc+)

a

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Cur

rent

Den

sity

(mA

/cm

2 )

Potential (V vs. Fc/Fc+)

a

g

a

g

80

Figure 4-5 Cyclic voltammograms of the SprayDOTTM-Green 145 in 0.1 M TBAP/PC at scan rates of (a) 25, (b) 50, (c) 75, (d) 100, (e) 150, (f) 200 and (g) 300 mV/s, on a Pt-button electrode. Film was prepared by drop-casting onto a Pt-button electrode from a 2 mg/mL polymer/toluene solution.

4.1.3 Spectroelectrochemistry

Spectroelectrochemistry was used to assess the electronic structure and the nature of

electrochromism in conducting polymers since polarons, bipolarons and π-dimers all factor into

the overall properties of organic EC polymers. Spectroelectrochemical and colorimetric studies

were conducted to acquire the optical characteristics of the polymer films in the correct potential

ranges determined by the CV experiments. The spectroelectrochemical series for polymer films

are shown in Figures 4-6 to 4-8. In their neutral states, SprayDOTTM-Purple 101 appears purple

(absorbing mainly at 574 nm), SprayDOTTM-Green 145 appears green (absorbing at 465nm and

707 nm) and PProDOP-N-EtCN appears very transmissive (transmitting along the full visible

region). As these polymer films are doped, charge carrier states emerge with the majority of the

light absorption for each polymer being in the near infrared, which results in highly transmissive

films. In case of SprayDOTTM-Green 145, going from -0.35 to +0.55 V (vs. Fc/Fc+), %T values

for lower band (at 465 nm) increases from 12% to 64%, resulting in a 52% change in %T. But,

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

-8

-6

-4

-2

0

2

4

6

8

10

Cur

rent

Den

sity

(mA

/cm

2 )

Potential (V vs. Fc/Fc+)

g

a

a

g

81

this change in %T is limited to 36% at higher band (at 707 nm), starting from 4% in the neutral

state increasing to 40% as it is doped. This lower contrast at the higher band results in a light

blue hue in the trasnmissive state appearance of the film.(Figure 4-7) With green polymers while

the interband transition bleaches during oxidative doping there is a lower energy carrier band that

tends to tail into the region of the initial band, so the truly bleached state is never observed. On

the other hand, PProDOP-N-EtCN appears transmissive in the entire visible region at every

redox state, even though it is proved to be electrochemicaly active (Figure 4-8). The interesting

behavior of PProDOP-N-EtCN is attributed to the conformational changes that lead to changes in

the conjugation length. While bulky substituents disrupt planarity, lower conjugation length and

induce tailing into the visible region, small N-substituents allow for free rotation in the neutral

state, thus the onset of the π-π* transition is located at the boundary of the visible and ultraviolet

regions of the spectrum. As such, the polymer is colorless in the neutral state. Bipolaron

undergoes like-charge repulsions and because of small N-substitution the polymer is free to

rotate into planarity as well. This red-shifts the bipolaron peak further into the NIR. Moreover,

the substitution which allows rotation and positioning of alkyl groups above and below the plane

of the π-conjugated chain can allow high doping levels and inhibit π-stacking. More open and

less dense polymer morphology will allow more dopant ion accommodation in the film resulting

in higher transmittance due to higher doping.

4.1.4 Setting Thicknesses

The effect of thickness on color and optical contrast is inevitable. In order to study this

effect and be able to pick the optimum thicknesses for the EC polymers that are to be utilized in

devices, we have generated absorbance vs. thickness calibration plots. For this purpose, 8 films

of both SprayDOTTM-Purple 101 and SprayDOTTM-Green 145 were sprayed on ITO/glass. Each

film had a different absorbance value ranging from 0 to 2 (in a.u. at λmax). The film thicknesses

82

Figure 4-6 Spectroelectrochemistry of spray-cast, redox switched, SprayDOTTM-Purple 101 film at applied potentials of (a) -0.35 (b) -0.30 (c) -0.25 (d) -0.20 (e) -0.15 (f) -0.10 (g) -0.05 (h) 0.00 (i) +0.05 (j) +0.10 (k) +0.15 (l) +0.25 (m) +0.30 and (n) +0.35 V vs. Fc/Fc+.

Figure 4-7 Spectroelectrochemistry of spray-cast, redox switched, SprayDOTTM-Green 145 film at applied potentials of (a) -0.35 to (s) +0.55 V vs. Fc/Fc+ at intervals of 50 mV/s. Inset: Change in %T (absorbance) at two peak wavelengths, 465 nm and 707 nm, by applied potential.

400 600 800 1000 1200 1400 16000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

n

a

a

Abs

orba

nce

(a.u

.)

wavelength (nm)

n

a

400 600 800 1000 1200 1400 16000.00.20.40.60.81.01.21.41.61.82.02.22.42.6

s

a

s

Abs

orba

nce

(a.u

.)

wavelength (nm)

a

b

-0.6 -0.3 0.0 0.3 0.60

15

30

45

60

75

40%

64%

12%4%

465 nm

% T

rans

mitt

ance

Potential (V vs. Fc/Fc+)

707 nm

83

Figure 4-8 Spectroelectrochemistry of potential-scan deposited (10 scans, at 50 mV/s, from -0.63 to +0.57 V vs. Fc/Fc+), redox switched, PProDOP-N-EtCN film at applied potentials of (a) -0.08 to (h) +0.27 V vs. Fc/Fc+ at intervals of 50 mV/s. Inset: Scan rate dependence of the PProDOP-N-EtCN on Pt button electrode in 0.1 M TBAP/PC.

were measured by a profilometer after each film was conditioned by potential stepping between

its reduced and oxidized states. Since spray cast films are rough, 5 measurements were made on

each film and the average of these data were used for Absorbance (a.u.) vs. thickness (A0) plots.

In each case the thickness scaled linearly with Absorbance, allowing us to correlate film

thickness to absorbance by linear fit equations as shown in Figure 4-9.

Figure 4-9 Absorbance (a.u.) vs. thickness (Ao) linear fit plots. (a) SprayDOTTM-Purple 101 (λmax=574 nm) and (b) SprayDOTTM-Green 145 (λmax=707 nm)

400 600 800 1000 1200 1400 16000.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

h

Abs

orba

nce

(a.u

.)

wavelength (nm)

a-0.3 -0.2 -0.1 0.0 0.1 0.2

-1.5-1.0-0.50.00.51.01.5

g

a

g

mA

/cm

2

V vs. Fc/Fc+

a

0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

1500

3000

4500

6000

7500

9000

Film

thic

knes

s (A

0 )

Absorbance (a.u.)

y = 3715 x + 1219R2 = 0.95

a

0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.001000

2000

3000

4000

5000

6000

Film

thic

knes

s (A

o )

Absorbance (a.u.)

y = 2725 x + 606R2 = 0.93

b

84

4.1.5 Tandem Chronocoulometry and Chronoabsorptometry

Polymer films with similiar switching times and giving the highset contrast ratios were

chosen for application in to the dual-film technique, P3-ECD and 3-ECD. A tandem

chronoabsorptometry/chronocoulometry experiment was used to calculate composite coloration

efficiencies and switch times at 95% of the total optical change (%T) at λmax.19, 21, 65 Coloration

efficiencies of the films at specific thicknesses are plotted in Figures 4-10 to 4-12. The effect of

film thickness on coloration efficiency and switch time is tabulated in Table 4-1. Since

SprayDOTTM-Green 145’s enhanced electrochromism is not limited to a single wavelength, the

coloration efficiency was calculated at both λmax. CE values of SprayDOTTM-Purple 101 are

higher than SprayDOTTM-Green 145’s CE values. The bulkier structure of SprayDOTTM-Green

145 holds more charge than SprayDOTTM-Purple 101 and has a lower optical contrast due to the

enhanced tailing of the NIR absorption in to the red absorbing region in the conductive state.

The SprayDOTTM-Purple 101 film with a thickness of 500 nm has a charge density of 1.1

mC/cm2 with a change in %T of 52% resulting in a CE of 707 cm2/C at λmax of 574 nm (Figure

4-10), whereas, the thinner SprayDOTTM-Green 145 film (380 nm thick) has a charge density of

1.8 mC/cm2 with a change in %T of 48% resulting in a CE of 299 cm2/C at λmax of 465 nm

(Figure 4-11) and a change in %T of 39% resulting in a CE of 430 cm2/C at λmax of 707 nm

(Figure 4-12). Since PProDOP derivatives (PProDOP-N-EtCN) and nitroxyl radical polymers

(PTMA) have unsaturated pastel colors or no colors, the optical properties of the dual or multi

EC systems are dominated by the saturated colors of cathodically coloring polymers such as

PProDOT derivatives.

Switch times are regulated by the diffusion of the counterions through the films during

redox switching. Enhanced optical response times (in seconds) were observed in these systems

since the open morphology of the polymer (bulky structures) promotes the mobility of charge

85

compensating counterions. The switch times strongly depend on the thickness of the films, thus

thinner films bleach in subseconds while the thicker ones bleach in longer times. (Table 4-1). In

contrary to switch times, the coloration efficiency is shown to be independent of film thickness

for these systems. This behavior is attributed to the compensation of the increase in change in

optical density, ΔOD,by the increase in charge density.

Figure 4-10 Tandem chronoabsorptometry and chronocoulometry experiment for 500 nm thick SprayDOTTM-Purple 101 (stepped from -0.2 V to +0.4 V vs. Fc/Fc+, held for 10 s at each potential at 574 nm) in 0.1 M TBAP/PC solution.

Figure 4-11 Tandem chronoabsorptometry and chronocoulometry experiments for 380 nm thick SprayDOTTM-Green 145 stepped from -0.30 to +0.60 V vs. Fc/Fc+, held for 10 s at each potential at 465 nm in 0.1 M TBAP/PC solution.

0 2 4 6 8 10 12 14 16 18 200.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

time (sec.)

Cha

rge

Den

sity

(mC

/cm

2 )at 574 nm

10

20

30

40

50

60

70

% Transm

ittance

0 2 4 6 8 10 12 14 16 18 20

0.5

1.0

1.5

2.0

2.5

3.0

time (sec.)

Cha

rge

Den

sity

(mC

/cm

2 )

20

30

40

50

60

70

% Transm

ittance

at 465 nm

86

Figure 4-12 Tandem chronoabsorptometry and chronocoulometry experiments for 380 nm thick SprayDOTTM-Green 145 stepped from -0.30 to +0.60 V vs. Fc/Fc+, held for 10 s at each potential at 707 nm in 0.1 M TBAP/PC solution.

Table 4-1 Coloration efficiencies and switch times of SprayDOTTM-Purple 101 and SprayDOTTM-Green 145 at various film thicknesses in 0.1 M TBAP/PC. Values are reported at 95% of full switch.

Green 145 Purple 101 Absorbance (a.u.) Thickness (nm)

CE at 465nm t0.95

a CE at 707 nm

t0.95a

Absorbance (a.u.) Thickness (nm)

CE at 574 nm t0.95

a

0.42 a.u. 180 nm

333 cm2/C 1.2 s

485 cm2/C 0.4 s

0.19 a.u. 190 nm

651 cm2/C 1.0 s

0.59 a.u. 220 nm

291 cm2/C 2.3 s

317 cm2/C 1.3 s

0.64 a.u. 360 nm

758 cm2/C 2.0 s

0.84 a.u. 290 nm

284 cm2/C 1.5 s

414 cm2/C 1.2 s

0.93 a.u. 470 nm

692 cm2/C 1.4 s

1.17 a.u. 380 nm

299 cm2/C 1.0 s

430 cm2/C 1.2 s

1.03 a.u. 500 nm

707 cm2/C 2.2 s

1.68 a.u. 520 nm

237 cm2/C 2.1 s

568 cm2/C 1.8 s

1.40 a.u. 640 nm

855 cm2/C 1.6 s

1.82 a.u. 560 nm

296 cm2/C 2.1 s

428 cm2/C 2.3 s

1.79 a.u. 790 nm

780 cm2/C 2.9 s

a Switch times for 95% of full switch. 4.1.6 Colorimetry

The effect of thickness on color and contrast was studied by recording % relative

luminance values at applied potentials from films with various thicknesses. As shown in Figures

0 2 4 6 8 10 12 14 16 18 20

0.5

1.0

1.5

2.0

2.5

3.0

time (sec.)

Che

rge

Den

sity

(mC

/cm

2 )10

20

30

40

50

% Transm

ittance

at 707 nm

87

4-13 and 4-14, the luminance contrast (C) and color contrast (ΔE) increases by increasing film

thickness. For SprayDOTTM-Purple 101, as the thickness increases from 190 nm to 790 nm the

luminance contrast increases from 0.2 to 0.8 and the color contrast increases from 17 to 78. For

SprayDOTTM-Green 145, as the thickness increases from 150 nm to 520 nm the luminance

contrast increases from 0.1 to 0.5 and the color contrast increases from 14 to 38. The contrast

was observed to increase as the films thickness increases and level off. Upon doping, the

bipolaron carrier bands shift into the NIR region by repressing the absorbance in the visible

region, thus the optical contrast increases. Thicker films of sterically bulk structures can

incorporate more counter ions to compensate for the oxidation, therefore, as the thickness

increases, polymers high luminance at the oxidized state maintains and the luminance in the

reduced state decreases.

Figure 4-13 % Relative Luminance as a function of applied potential of SprayDOTTM-Purple 101 film at thicknesses of -■- 190 nm (0.19 a.u.), -●- 360 nm (0.64 a.u.), -▲- 500 nm (1.03 a.u.), -♦- 640 nm (1.40 a.u.), -◄- 790 nm (1.79 a.u.) in 0.1 M TBAP/PC.

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.50

102030405060708090

100

790 nm

190 nm

C=0.8C=0.8C=0.6

C=0.2

% R

elat

ive

Lum

inan

ce

Potential (V vs. Fc/Fc+)

a

C=0.5

88

Figure 4-14 % Relative Luminance as a function of applied potential of SprayDOTTM-Green 145 film at thicknesses of -■- 150 nm (0.34 a.u.), -●- 220 nm (0.60 a.u.), -▲- 320 nm (0.97 a.u.), -♦- 450 nm (1.42 a.u.), -◄- 520 nm (1.70 a.u.) in 0.1 M TBAP.

4.1.7 Optical Stability

One of the major aspects of ECDs is long-term optical stability in ambient conditions.

Therefore, optical stabilities of the EC polymers were investigated prior to device constructions.

The stability experiments were performed by stepping the potential between the extreme redox

states of SprayDOTTM-Purple 101 and SprayDOTTM-Green 145 films (sprayed on ITO/glass) in

0.1 M TBAP/PC. In order to express the change in the color as well as the luminance, the

lightness, L*, of the films at their neutral states were recorded over time. The charge densities,

Qred, associated with the neutralization of the films were also recorded along the time.

SprayDOTTM-Purple 101 film on ITO/glass retained 50% of its optical response after 5000

double potential steps while SprayDOTTM-Green 145 retained 70% of its optical response after

15 000 double potential steps.(Figure 4-15) The redox reaction of molecular oxygen in air is

accepted as the threshold for air stability. (E (O2/H2O)= 0.5 V vs. SCE or 0.12 V vs Fc/Fc+) 60, 61,

72 Since the onset of oxidation for SprayDOTTM-Purple 101 and SprayDOTTM-Green 145 (-0.2

and -0.3 V vs. Fc/Fc+, respectively-obtained from polymer CVs) are below the threshold for air

-0.4 -0.2 0.0 0.2 0.4 0.60

102030405060708090

100

150 nm

520 nm

C=0.5C=0.5C=0.4

C=0.2

% R

elat

ive

Lum

inan

ce

Potential (V vs. Fc/Fc+)

b

C=0.1

89

stability these films tend to maintain their color (neutral state) at ambient conditions. Therefore,

the minor fading of the colors of the films at long-term switching studies is ascribed mainly to

the electrochemical dissolution of the films and partly to the accumulation of high levels of O2 in

the medium at long switch times. Over oxidation might also be responsible for the optical loss.

Since both polymer films don’t show any optical loss to a certain cycle (SprayDOTTM-Purple

101 is stable up to 2500 cycles and SprayDOTTM-Green 145 is stable up to 6000 cycles) we can

speculate on formation of double bonds between S on thiophene units and O in air. This

irreversible reaction results in a sudden optical loss. The degree of over oxidation affects the

degree of optical and charge loss.

Figure 4-15 Electrochemical and optical stability of (a) SprayDOTTM-Purple 101(thickness 410 nm, 8 s delay time) and (b) SprayDOTTM-Green 145 (thickness 380 nm, 7 s delay time) films on ITO/glass in 0.1 M TBAP/PC solution. (-●- L*, Lightness at neutral state and -▲- Qred, charge density)

4.2 SprayDOT-Purple 101/SprayDOT-Green 145 Dual-Film Electrochromic System

The dual EC-film film characterization method was utilized to predict the colors that could

be generated by multi-electrode devices whose working principles are based on color mixing.

Both, SprayDOTTM-Green 145 and SprayDOTTM-Purple 101 on ITO electrodes under separate

potentiostatic control were placed back to back in a 1cm quartz cell as to serve as two working

0 1000 2000 3000 4000 50000

20

40

60

80

100

# of redox cycles

Ligh

tnes

s (L

*)

0.5

1.0

1.5

2.0

2.5

3.0

Qred (m

C/cm

2)

a

0 3000 6000 9000 12000 150000

20

40

60

80

100

# of redox cycles

Ligh

tnes

s (L

*)

0

1

2

3

4

5

6

Qred (m

C/cm

2)

b

90

electrodes, with a Ag wire as a reference electrode and a Pt wire as a counter electrode. In situ

color coordinates and electromagnetic spectra in the visible region were recorded from the dual-

polymer system upon application of different potentials to different working electrodes in a 0.1

M TBAP/PC.73

In order to get an insight of the addition of optical properties by means of the dual-film

EC, absorbance spectra of SprayDOTTM-Purple 101 and SprayDOTTM-Green 145 films, were

taken separately at reduced (both at -0.66 V vs. Fc/Fc+) and oxidized (both at +0.74 V vs. Fc/Fc+)

states and these spectra were summed theoretically. This is demonstrated by the single film

purple and green curves in Figure 4-16, along with the theoretical summation represented by the

red curve. Black curve represents the spectral response of films combined in dual-EC method.

The theoretical and experimental spectra are shown to overlay one another for the oxidized and

reduced films.

The dual-film system was studied colorimetrically. The photographs and the L*a*b color

coordinates as a function of the separate potential applied to each film are shown in Figure 4-17.

The film thicknesses were set by utilizing the contrast data and plots that were reported in earlier

sections, thus the optimum contrast is obtained from the dual-EC system in addition to full

palette of colors. As it was shown in Figures 4-13 and 4-14 the maximum contrast that could be

obtained from SprayDOTTM-Purple 101is 0.8 while this value is only 0.5 for SprayDOTTM-Green

145. Therefore, SprayDOTTM-Purple 101 with a thickness of 605 nm (A=1.30 a.u.) switching

from deep purple (L*= 41, a*= 22, b*= -48, Q= 67) to highly transmissive sky blue (L*= 87, a*=

-2, b*= -7, Q= 87) upon doping with C= 0.71 and ΔE = 66 and SprayDOTTM-Green 145 with a

thickness of 360 nm (A=1.10 a.u.) switching from deep green (L*= 60, a*= -23, b*= 12, Q= 65)

91

to highly transmissive sky blue (L*= 84, a*= -4, b*= -6, Q= 84) open doping with C= 0.40 and

ΔE = 36 were utilized in multi-electrode systems. (Figure 4-17)

Figure 4-16 UV–vis-NIR spectra of SprayDOTTM-Purple 101/SprayDOTTM-Green 145 from dual-polymer electrochromic setup at (a) reduced and (b) oxidized states in 0.1 M TBAP/PC solution.

The new color palette we established extends from deep blue-black (L*= 21, a*= 3, b*= -

28, Q= 35) to clear (L*= 75, a*= -6, b*= -12, Q= 76) and embodies all tones of mixtures of green

400 600 800 1000 1200 1400 16000.0

0.5

1.0

1.5

2.0

SprayDOT-Purple 101 SprayDOT-Green 145 theoretical summation of

SprayDOT-Purple 101 and SprayDOT-Green 145

experimantal summation of SprayDOT-Purple 101 and SprayDOT-Green 145

Abs

orba

nce

(a.u

.)

wavelength (nm)

a

400 600 800 1000 1200

0.0

0.5

1.0

1.5

2.0 SprayDOT-Purple 101 SprayDOT-Green 145 theoretical summation of

SprayDOT-Purple 101 and SprayDOT-Green 145

experimantal summation of SprayDOT-Purple 101 and SprayDOT-Green 145

Abs

orba

nce

(a.u

.)

wavelength (nm)

b

92

and purple. The luminance contrast for this color gamut is 0.90 and the color contrast is 69. It is

proven that color mixing improves both the luminance,C, and color contrast, ΔE. Furthermore,

having two colors mixed and absorbance extended along the full visible range (formation of

black) any hindrance to contrast due to the chroma is eliminated to noticeable extent.

Figure 4-17 L*a*b* color coordinates and photography for SprayDOTTM-Purple 101/SprayDOTTM-Green 145 in 0.1 M TBAP/PC (all potentials are reported vs. Fc/Fc+)

4.3 Pseudo-Three-Electrode ECD (SprayDOTTM-Purple 101/SprayDOTTM-Green 145/PProDOP-N-EtCN)

We applied dual-EC method to electrochromic devices. P3-ECD that we propose can be

utilized either as a window type device or as a display by use of a reflective background such as

-0.66 V -0.46 V -0.26 V -0.06 V 0.14 V 0.34 V 0.54 V 0.74 V

L*=60a*=-23b*=12

L*=60a*=-24b*=12

L*=64a*=-15b*=3

L*=68a*=-11b*=-4

L*=76a*=-6b*=-8

L*=78a*=11b*=-8

L*=82a*=3b*=-6

L*=84a*=-4b*=-6

-0.66 L*=41a*=22b*=-48

L*=21a*=3b*=-28

L*=22a*=0b*=-27

L*=22a*=10b*=-33

L*=27a*=8b*=-36

L*=28a*=15b*=-44

L*=30a*=18b*=-48

L*=31a*=19b*=-49

L*=31a*=22b*=-51

-0.46 L*=41a*=23b*=-49

L*=22a*=5b*=-29

L*=22a*=6b*=-30

L*=23a*=8b*=-34

L*=25a*=14b*=-40

L*=27a*=16b*=-45

L*=30a*=17b*=-48

L*=31a*=18b*=-49

L*=32a*=18b*=-50

-0.26 L*=45a*=21b*=-43

L*=23a*=3b*=-27

L*=24a*=3b*=-28

L*=25a*=6b*=-32

L*=26a*=6b*=-35

L*=27a*=16b*=-44

L*=30a*=19b*=-48

L*=32a*=18b*=-48

L*=32a*=21b*=-50

-0.06 L*=60a*=17b*=-23

L*=28a*=-1b*=-20

L*=29a*=0b*=-20

L*=30a*=1b*=-24

L*=33a*=8b*=-31

L*=36a*=11b*=-36

L*=38a*=14b*=-39

L*=40a*=15b*=-39

L*=41a*=13b*=-39

0.14 L*=79a*=4b*=-4

L*=42a*=-9b*=-3

L*=42a*=-7b*=-4

L*=43a*=-3b*=-8

L*=45a*=0b*=-13

L*=50a*=4b*=-18

L*=54a*=5b*=-19

L*=56a*=8b*=-19

L*=58a*=8b*=-19

0.34 L*=82a*=14b*=-3

L*=51a*=-20b*=7

L*=52a*=-18b*=5

L*=54a*=-11b*=-1

L*=58a*=-9b*=-6

L*=63a*=-4b*=-11

L*=67a*=-3b*=-12

L*=70a*=-3b*=-11

L*=71a*=-3b*=-11

0.54 L*=86a*=0b*=-5

L*=54a*=-21b*=5

L*=54a*=-20b*=5

L*=56a*=-16b*=0

L*=59a*=-13b*=-6

L*=65a*=-9b*=-12

L*=70a*=-6b*=-13

L*=73a*=-5b*=-12

L*=74a*=-4b*=-12

0.74 L*=87a*=-2b*=-7

L*=55a*=-22b*=6

L*=55a*=-21b*=4

L*=57a*=-15b*=-1

L*=61a*=-11b*=-7

L*=66a*=-8b*=-12

L*=71a*=-6b*=-13

L*=74a*=-6b*=-12

L*=75a*=-6b*=-12

Purple 101

Green 145vs Fc/Fc+

93

a cellulose paper or white paint. Physically the construction of the P3-ECD is very similar to

dual absorptive/transmissive windows in such that it consists of two absorptive/transmissive

windows in series whose counter electrodes are shorted. (Figure 4-18) SprayDOTTM-Green 145,

SprayDOTTM-Purple 101 and PProDOP-N-EtCN films on ITO electrodes were prepared as

described before. PProDOP-N-EtCN, non-color changing counter electrode polymer, was

electropolymerized on to ITO-coated glass slides and heated as detailed in the

spectroelectrochemistry section. The thicknesses of the non-color changing PProDOP-N-EtCN

films on the counter electrodes were set to ensure the charge balance with the polymer it was

facing and retain high transmissivity. After 10 potential scans, a highly transparent polymer

film, which has the capability to balance charges, was obtained. Further scans result in thicker

films with less transmissivity. PProDOP-N-EtCN films were dried at 55 oC under vacuum for 30

minutes. All polymer films were electrochemically conditioned by sweeping the potential.

Cathodically coloring SprayDOTTM-Green 145 and SprayDOTTM-Purple 101 were fully oxidized

(brought to their transparent state) and non-color changing films of PProDOP-N-EtCN were fully

neutralized to improve the charge balance prior to assembling of the device. The SprayDOTTM-

Green 145 and SprayDOTTM-Purple 101 films were then coated with gel electrolyte and then the

PProDOP-N-EtCN films were closed on top of them. Two devices were connected in series so

that the counter electrode of each device is back-to-back and connected with a copper tape to

serve as a conjunct counter electrode to the whole device. The SprayDOTTM- Purple 101 serves

as a working electrode-1 and the SprayDOTTM- Green 145 serves as working electrode 2. The

devices were encapsulated by paraffin wax and epoxy to allow long-term testing. In situ color

coordinates were recorded from the P3-ECD upon application of different potentials to different

working electrodes.

94

Figure 4-18 Schematic of the P3-ECD under bipotentiostatic control and a side view of the device

The absorbance spectra and the colorimetric data with photographs obtained from the first

P3-ECD device is shown in Figures 4-19 and 4-20. When a potential of +1.4 V is applied to

both WE1 (SprayDOTTM- Purple 101) and WE2 (SprayDOTTM- Green 145), the device becomes

highly transmissive (L*= 75, a*= -7, b*= -7) and when a potential of -0.2 V is applied to both

working electrodes, both films become absorptive and the device appears blue-black absorbing

along the full visible region (L*= 26, a*= -3, b*= -17) with a λmax of 582 nm. When a potential

of -0.2 V is applied to WE1 (SprayDOTTM- Purple 101) and +1.4 V to WE2 (SprayDOTTM-

Green 145), green film becomes transmissive and having purple in neutral state the device

appears deep purple (L*= 56, a*= 7, b*= -15). When a potential of +1.4 V is applied to WE1

(SprayDOTTM- Purple 101) and -0.2 V to WE2 (SprayDOTTM- Green 145), green film becomes

absorptive and purple becomes transmissive leading the device to appear deep green (L*= 57,

a*= -17, b*= 4). The P3-ECD device shows a luminance contrast of 0.82 and a color contrast of

50.

WE 2

WE 1

CE

o

observer

WE 1 WE 2

CE

95

Figure 4-19 UV–vis-NIR spectra of the P3-ECD. Working electrode 1 (coated with SprayDOTTM-Purple 101) reduced (purple line), working electrode 2 (coated with SprayDOTTM-Green 145) reduced (green line), both of the working electrodes reduced (black line) and both working electrodes oxidized (blue line).

Figure 4-20 L*a*b* color coordinates and photography from the SprayDOTTM-Purple 101/SprayDOTTM-Green 145 P3-ECD

P3-ECD device having a reasonable contrast value between the darkest and the lightest

states beats other devices utilized in information display technology by offering a full color

400 600 800 1000 1200 1400 16000.0

0.5

1.0

1.5

2.0

Abs

orba

nce

(a.u

.)

wavelength (nm)

582 nm

L*= 60a*= -23b*= 12

L*= 84a*= -4b*= -6

L*= 41a*= 22b*= -48

L*= 26a*= -3b*= 17

L*= 56 a*= 7b*= -15

L*= 87a*= -2b*= -7

L*= 57a*= -17b*= 4

L*= 75a*= -7b*= -7

Purple 101

Green 145

96

palette. Other than that, the device shows a reasonable optical stability. In order to study that,

the device was potential stepped between its darkest and brightest states. %T was recorded at

582 nm over time. While bipotentiostat was utilized to supply potential to the device in the

earlier parts, two computer controlled potentiostats were utilized for long term studies. The P3-

ECD retained 50 % of its optical contrast after 1000 deep potential cycles. (Figure 4-21) The

counter electrode material PProDOP-N-EtCN with and oxidation onset of -0.15 V vs Fc/Fc+ is

stable in air, but still more prone to oxidation when compared to the other components of the

device. The oxidation of counter electrode material could render the charge compensation that is

paramount for device function. If the O2 penetration to the device can be eliminated by a better

insulation or if the device is constructed in an inert medium, longer switch times can be attained.

Figure 4-21 P3-ECD stability studies. %Transmittance over time was recorded from the SprayDOTTM-Purple 101/SprayDOTTM-Green 145 P3-ECD.

0 500 1000 1500 2000 25000

10

20

30

40

50

% T

rans

mitt

ance

(at 5

82 n

m)

# of redox cycles

32%16%

97

4.4 Three-Electrode ECD (SprayDOT-Purple 101/SprayDOT-Green 145/PTMA)

The P3-ECD was a device composed of two dual EC window type devices connected in

series and the outcome of the result was the summation of the optical properties of all

components at a given state. Here, we propose a three-electrode electrochromic display device,

3-ECD, which combines two cells in the P3-ECD into one single cell. A schematic of the 3-

ECD is given in Figure 4-22. The 3-ECD includes two transmissive working electrodes coated

with EC polymers coupled to a counter electrode that has an electroactive material that exhibits

little to no optical transition in the visible region, and a gel electrolyte to separate the layers to

provide contact between them. Electrical potentials can be independently applied between the

first working electrode and the counter electrode and second working electrode and the counter

electrode. The device displays a color depending upon the combination of colors seen through

the two working electrodes, whose colors depend on the potentials that are independently

applied.

Figure 4-22 Schematic of the 3-ECD under bipotentiostatic control

WE 2

WE 1

CE

WE 1 WE 2CE

observer

98

The unique aspect of this device is the counter electrode, sandwiched between the working

electrodes, which is porous to allow electrolyte transfer, highly transparent as to not limit optical

contrast, and conductive on both sides to supply sufficient charge to both working electrodes for

complete redox reactions. The counter electrode we developed in this work, of which the

schematic is given in Figure 4-23, has three components, polyester membrane, PEDOT:PSS and

gold. Track-etched polyester membrane (10 micron pore diameter), PETE, serves as the porous

support. In order to provide conductivity in 3D, high conductivity PEDOT:PSS formulation, was

spin-coated on both sides of the membrane at 3000 rpm for 30 sec., and the films were dried

under vacuum at 120 oC for 2 hours. As the thickness of the PEDOT:PSS increased on PETE,

the conductivity increased at the expense of transmittance in the visible range. One layer of

PEDOT:PSS results in a surface resistivity of 1000 Ω/□, while three layers result in 320 Ω/□ and

four layers in 160 Ω/□. As an approximation of change in conductivity across the film,

resistance measurements were made with contacts on each film side. Due to the porosity of the

film, these contacts could not be perfectly opposed, making exact conductivity measurements

difficult. It was seen that the through film resistance decreases from 1100 Ω/□ to 400 Ω/□ and

finally to 200 Ω/□ as the number of layers of PEDOT:PSS is increased. Increasing the number

of PEDOT:PSS layers decreased the film quality since comet-like structures appeared due to the

accumulation of particles. Uneven counter electrode films disrupted the final appearance of the

devices. Therefore, a thin layer of Au was utilized. PETE coated with only 1.5 nm Au on each

side had surface resistivity in the range of MΩ/□ and showed no conductivity across the film.

Increasing the thickness of Au layer increased the conductivity but films became highly

reflective. We have decided to utilize Au to form low resistance clusters on the surface of the

film and made the contacts through these clusters by PEDOT:PSS. Evaporation of 1.5 nm of Au

99

on PETE which was already coated with one layer of PEDOT:PSS on each side decreased the

surface resistance from 1000 Ω/□ to 500 Ω/□. The conductivity across the film also decreased

from 1100 Ω/□ to 500 Ω/□.

Figure 4-23 Schematic of the highly transmissive porous electrode (PETE/Au/PEDOT:PSS)

In Figure 4-24, transmisivity of the counter electrode material components is studied

systematically. All counter electrode components were sandwiched between glass slides with a

gel electrolyte for %Transmittance measurements. ITO shows an average transmittance of ~87%

along the visible region with a surface resistance of 8-10 Ω/□. Nonconductive PETE membrane

shows an average transmittance of ~75%. When a layer of PEDOT:PSS is deposited on PETE,

the surface resistance decreases to 1000 Ω/□ with a negligible loss in transmittance. As the

number of PEDOT:PSS increases the resistance decreases to 150-200 Ω/□ and transmittance also

decreases to ~60%. When 1.5 nm Au was evaporated to both sides of PETE it shows a

resistance in the range of M Ωs and an average transmissivity of 67% with a minimum of 60% at

550 nm. When 1.5 nm Au is evaporated on PETE, coated with one layer of PEDOT:PSS on

each side, the transmissivity goes down to 57% with a minimum at 55% while the resistance

decreases from 800 Ω/□ to 500 Ω/□.

1 layer PEDOT:PSS

500 Ω/□

500 Ω/□

Track etched PETE membrane

1.5 nm Au

100

Figure 4-24 Systematic % Transmittance study of counter electrodes/counter electrode components, ITO, PETE, PETE/Au, PETE/PEDOT:PSS and PETE/Au/PEDOT:PSS. (All materials were covered with gel electrolyte and sandwiched between glass slides for measurements)

The transmissive porous electrode, PETE/PEDOT:PSS/Au was coated with an

electroactive polymer layer that does not change color yet undergoes an electrochemical redox

reaction and acts to balance charge during switching in 3-ECD. In this work we employed a

nitroxide radical polymer, PTMA. PTMA is a polymethacrylate derivative with a 2,2,6,6-

tetramethyl-1-piperidinyloxy, TEMPO, stable free-radical in the repeat unit. TEMPO containing

compounds are known to have redox behavior and they find application in batteries due to their

high capacities of charge storage.74-77 We use PTMA as a charge storage material on our counter

electrodes. The electrochemical properties of PTMA was investigated in order to set the baseline

conditions prior to the device construction. PTMA solution (2mg polymer/1mL toluene) was

drop cast on to the Pt-button electrode after being filtered. After the film was air-dried, it was

immersed in to the 0.1 M TBAP/PC solution for scan rate studies. The potentials were swept at

different scan rates. As shown in Figure 4-25, the peak currents increases linearly with

increasing scan rates indicating the existence of well-adhered electroactive film. The redox

400 600 800 1000 1200 1400 16000

102030405060708090

100

% T

rans

mitt

ance

wavelength (nm)

PETE PETE/Au PETE/PEDOT:PSS/Au 1 PEDOT:PSS 3 PEDOT:PSS 4 PEDOT:PSS ITO

101

reaction of PTMA takes place in a 0.5 V potential window and the anodic oxidation of the stable

nitroxide radical results in an oxoammonium cation. (Figure 4-26) The sharp increase in current

(sharp and narrow peaks) is indicative of fast electron movements.

Figure 4-25 Cyclic voltammograms of PTMA in 0.1 M TBAP/PC at scan rates of (a) 25, (b) 50, (c) 75, (d) 100, (e) 150, (f) 200 and (g) 300 mV/s, on a Pt-button electrode. Film was prepared by drop casting from 2mg/mL polymer/toluene solution.

Figure 4-26 Redox couples of PTMA.

While the PTMA adhered to the Pt button and gave a sufficient redox behavior, it showed

a weak adherence to ITO/glasss. The PTMA was sprayed onto ITO/glass from a 15 mg

PTMA/60 mL toluene solution and air dried. Then the film was immersed in to the 0.1 M

LiClO4/PC for cyclic voltammetry characterizations. As the potential was swept between the

extreme states the film dissolved into the solution which is seen by a decrease in the peak current

-0.1 0.0 0.1 0.2 0.3 0.4 0.5

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

Cur

rent

Den

sity

(mA

/cm

2 )

Potential (V vs. Fc/Fc+)

a

g

a

g

102

by increasing number of scans. (Figure 4-27 (a)) In order to overcome this solubility, the PTMA

was blended with a high molecular weight PMMA in 1:4 weight ratio (15 mg PTMA/60 mg

PMMA/60 mL toluene) and sprayed onto ITO/glass and air dried. As seen in Figure 4-27 (b)

PTMA/PMMA blend film was stable upon potential scanning and did not dissolve in to the

electrolyte solution, but the electroactivity decreased by 50%, even there was the same amount of

electroactive polymer on the electrode. The PTMA/PMMA blend film was annealed at 90 oC,

under vacuum, for 1 hour, and cooled down slowly. The temperature 90 oC was chosen after

series of electrochemical studies which proved it to be the optimum temperature and because it is

lower than the decomposition temperatures of both PTMA and PMMA which are determined to

be 240 oC and 165 oC, respectively, by TGA studies and it is higher or equal to the glass

transition temperatures,Tg, of each polymer. (Tg (PTMA)= 76 oC and Tg(PMMA)= 105 oC )75

Complete melting of the polymer blend and reorganization upon cooling helped form more

continuous matrix and enhanced the charge mobility, hence electroactivity. (Figure 4-27 (c)).

These studies were repeated in different electrolytes such as TBAP/PC and TBAP/ACN in our

labs. Similar electrochemical behavior was observed. Studies in our labs also showed an

electrochemical stability of thousands of potential cycles for annealed PTMA/PMMA blends.

PTMA’s optical properties also make it a good candidate in use for counter electrodes. PTMA

films are transmissive over the entire visible range and show no change in absorbance upon

redox reactions as shown in Figure 4-28. When PTMAs are used on counter electrodes in ECDs

they do not limit the contrast, but supply charge and stability to devices.

103

Figure 4-27 PTMA formulation studies in 0.1 M LiClO4/PC. Films were spray cast on ITO/glass from (a) 15 mg PTMA/60 mL toluene solution and used as it is, (b) 15mg PTMA/60 mg PMMA/60 mL toluene solution and used as it is and, (c) 15mg PTMA/60 mg PMMA/60 mLsoluion and annealed at 90 oC under vacuum for 1 hour before use.

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

Cur

rent

Den

sity

(mA

/cm

2 )

Potential (V vs. Fc/Fc+)

2 10 20

a

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8-0.08

-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

Cur

rent

Den

sity

(mA

/cm

2 )

Potential (V vs. Fc/Fc+)

2 10 20

b

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8-0.16

-0.12

-0.08

-0.04

0.00

0.04

0.08

0.12

0.16

Cur

rent

Den

sity

(mA

/cm

2 )

Potential (V vs. Fc/Fc+)

2 10 20

c

104

Figure 4-28 Spectroelectrochemistry of spray cast, redox switched, PTMA/PMMA.film at applied potentials of -0.23 V to+0.37 V vs. Fc/Fc+ at intervals of 200 mV/s in 0.1 M TBAP/PC. Inset: Scan rate dependence of the PTMA on Pt button electrode in 0.1 M TBAP/PC.

The 3-ECD was constructed after the optimization of porous, transmissive counter

electrode, PETE/PEDOT:PSS/Au and the non-color changing, electroactive counter electrode

polymer layer, PTMA/PMMA/Heat. First, SprayDOTTM-Green 145 and SprayDOTTM-Purple

101 were spray cast onto ITO electrodes and dried under vacuum. Then, to provide enough

redox sites at the counter electrode, first, a layer of (2mL) PTMA solution was sprayed on to the

PETE/PEDOT:PSS/Au electrode, and then several layers of PTMA/PMMA blend solution was

sprayed until the film takes a milky opaque color (This opaque color disappears when the film is

inserted in a solution). Film was annealed in vacuum oven for an hour at 90 oC. Cathodically

coloring SprayDOTTM-Green 145 and SprayDOTTM-Purple 101 were fully oxidized (brought to

their transparent state) and non-color changing PTMA/PMMA blend films was fully neutralized

to improve the charge balance prior to assembling of the device. The SprayDOTTM-Green 145

400 600 800 1000 1200 1400 16000.0

0.2

0.4

0.6

0.8

1.0

Abs

orba

nce

(a.u

.)

wavelength (nm)

-0.1 0.0 0.1 0.2 0.3 0.4 0.5

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

Cur

rent

Den

sity

(mA

/cm

2 )

Potential (V vs. Fc/Fc+)

a

g

a

g

105

and SprayDOTTM-Purple 101 films were then coated with gel electrolyte and then the counter

electrode was sandwiched between them. The SprayDOTTM- Purple 101serves as a working

electrode 1 and the SprayDOTTM- Green 145 serves as working electrode 2 and the counter

electrode is utilized by both working electrodes. The device was encapsulated by a paraffin wax.

In situ color coordinates were recorded from the 3-ECD upon application of different potentials

to different working electrodes.

The absorbance spectra and the colorimetric data with photographs obtained from the first

3-ECD are shown in Figures 4-29 and 4-30. UV–vis-NIR spectra from SprayDOTTM-Purple

101/SprayDOTTM-Green 145 3-ECD shows a broad absorption with a λmax of 608 nm over the

entire visible range upon reduction of both EC polymers.(Figure 4-29 (a)) The device becomes

totally clear/transmissive upon doping of both EC layers. When a potential of 1.5 V is applied to

both WE1 (SprayDOTTM-Purple 101) and WE2 (SprayDOTTM-Green 145), films become highly

transmissive (L*= 62, a*= 0, b*= -1) and when a potential of -0.5 V is applied to both working

electrodes, both films become absorptive and the device appears blue-black absorbing along the

full visible region (L*= 25, a*= 7, b*= -1). When a potential of -0.5 V is applied to WE1

(SprayDOTTM- Purple 101) and +1.5 V to WE2 (SprayDOTTM- Green 145), green film becomes

transmissive and having purple in neutral state the device appears deep purple (L*= 35, a*= 21,

b*= -38). When a potential of +1.5 V is applied to WE1 (SprayDOTTM- Purple 101) and -0.5 V

to WE2 (SprayDOTTM- Green 145), green film becomes absorptive and purple becomes

transmissive leading the device to appear deep green (L*= 61, a*= -22, b*= 16). The 3-ECD

device shows a luminance contrast of 0.74 and a color contrast of 43.

106

Figure 4-29 UV–vis-NIR spectra of the 3-ECD. Working electrode 1 (coated with SprayDOT-Purple 101) reduced (purple line), working electrode 2 (coated with SprayDOT-Green 145) reduced (green line), both of the working electrodes reduced (black line) and both working electrodes oxidized (blue line). (a) Absorbance over the visible region, (b) absorbance over the UV-vis-NIR.

400 500 600 700 8000.0

0.1

0.2

0.3

0.4

0.5

Abs

orba

nce

(a.u

.)

wavelength (nm)

a

400 600 800 1000 1200 1400 16000.0

0.2

0.4

0.6

0.8

Abs

orba

nce

(a.u

.)

wavelength (nm)

b

107

Figure 4-30 L*a*b* color coordinates and photography from the SprayDOT-Purple 101/SprayDOT-Green 145 3-ECD.

4.5 Conclusions

In this work, we introduce the first two polymer black to transmissive and multi-colored

switching electrochromic display devices, P3-ECD and 3-ECD. To accomplish this, we have

developed a new transmissive, porous counter electrode which is composed of PETE,

PEDOT:PSS and Au. We also introduced the first use of non-electrochromic yet electroactive

polymers, PProDOP-N-EtCN and PTMA, as counter electrode materials in ECDs. These highly

transparent polymers help eliminate the contrast limitations in dual systems. P3-ECD and 3-

ECD add colors by transmitting light through two working electrodes (coated with two different

electrochromic polymer films) and a counter electrode/counter electrodes (coated with

PProDOP-N-EtCN or PTMA) stacked together with a gel electrolyte between the layers.

Further, the electrodes are all under separate potentiostatic control allowing the independent

control of color in each working electrode, thus wide range of color mixing. Utilizing

SprayDOTTM-Purple 101 and SprayDOTTM-Green 145 in the P3-ECD yield blue-black with

L*= 60a*= -23b*= 12

L*= 84a*= -4b*= -6

L*= 41a*= 22b*= -48

L*= 87a*= -2b*= -7

L*= 25 a*= 7 b*= -21

L*= 35 a*= 21 b*= -38

L*= 62 a*= 0 b*= -1

L*= 61 a*= -22 b*= 16

Purple 101

Green 145

108

color coordinates of L*= 26 a*= -3 b*= 17 switching to clear with color coordinates of L*= 75

a*= -7 b*= -7. The device had a high luminance contrast of 0.82. Utilizing SprayDOTTM-Purple

101 and SprayDOTTM-Green 145 in the 3-ECD yield blue-black with color coordinates of L*=

25 a*= 7 b*= -21 switching to clear with color coordinates of L*= 62 a*= 0 b*= -1. The device

had a high luminance contrast of 0.74 that is lower than the contrast obtained from P3-ECD. The

use of PEDOT:PSS and Au in the counter electrode adds a blue hue to the transmissive state of

3-ECD and limits the contrast of the device to a certain degree.

Separate potential control over the electrodes utilized the color mixing in these devices

showing that full palette of colors is accessible through smart choice of EC materials. Switches

in between any colors are possible independent of the existing state. By utilizing P3-ECD and 3-

ECD other than black and white scripts colored images can be displayed on information displays.

P3-ECD and 3-ECD stand out with their optical contrasts comparable to single films.

109

CHAPTER 5 RGB COLOR SPACE 5-ELECTRODE ELECTROCHROMIC DISPLAY DEVICE

In 1861 Maxwell demonstrated the first color photograph and initiated color display

technology. His demonstration was based on the trichromatic vision of the human eye. This

demonstration led to a great variety of color displays utilizing both additive and subtractive color

mixing.78

Color display technologies benefit from a human vision mishap and use the halftoning

principle. Halftoning is the conversion of continuous images into regular sequences of stripes,

dots or rectangles on displays or prints. The color that a human sees at a pixel is dependent on

the pixel and all the other pixels in the viewing angle. When humans view these patterns from a

distance the eye automatically adds the colors and the pattern appears as a continuous image.49, 78

As an example, the most widely used color display, cathode ray tube (CRTs), consists of a

regular sequence of red, green and blue dots, rectangles or stripes (pixels). These pixels are

made of inorganic compounds doped with metal ions, and referred to as phosphors. Red

contains yttrium oxysulfide (Y2O2S) doped with europium ions (Eu3+), while the green and blue

contain zinc sulfide doped with copper (Cu+) and silver (Ag+) ions, respectively. These pixels

are hit by electron beams whose direction and intensity is controlled by shadow masks and they

emit red, green and blue light which add up to new colors when viewed from a proper distance.53

The need for small volume and lower weight portable displays such as laptops, palm

computers, and navigation devices lead to the invention of flat panel displays. In flat panel

displays, liquid crystals sandwiched between transparent electrodes mimic a color filter. These

displays switch between opaque and transparent states upon the application of a potential. The

only drawback of these devices is their high costs and limited viewing angles.79 Electrochromic

polymers that switch from a colored to transmissive state upon application of potential could be

110

utilized as color filters and imitate the behavior of liquid crystals in transmitting/reflecting or

absorbing the light. Now having three primary color polymers, red, green and blue switching to

transmissive upon doping available we can develop an electrochromic full color display. It

should be understood that the working and color mixing principles of the full color ECDs we

propose are different from additive color mixing displays. The working principles of the RGB

color space 5-electrode ECD we propose are summarized in Figure 5-1. In the RGB 5-ECD, the

red, green and, blue polymers are sprayed onto transparent electrodes as color filters. When all

of the polymers are brought to their neutral-colored states the secondary subtractive colors red,

green and blue will absorb over the entire visible range, block the light, and the device will

appear black. The same effect in the same device could be obtained by mixing primary

subtractive colors cyan, magenta and yellow. The intensity of the transmitted light can be

controlled by precisely controlling the doping levels of the polymers. When all the films are

fully doped the device appears clear and transmits all the light.

In constructing devices certain details have to be taken into account. Since we will be

controlling the color by the potential applied (doping level) and the thicknesses of the films (the

intensity of transmitted light) we should set the conditions before the device is constructed.

Therefore, full electrochemical and optical characterizations had to be carried out prior to device

construction.

5.1 RGB Color Space 5-Electrode ECD-Fundamental Properties (SprayDOTTM-Red 252, SprayDOTTM-Green 179, SprayDOTTM-Blue 153, PTMA)

Alkoxy substituted thiophene copolymers with desirable properties, such as low bandgap

and low oxidation potentials were used in this work. Alternative additions of donor-acceptor

groups (alkoxy thiophenes and BTDs) in the repeat units resulted in polymers that absorb mainly

at two (high and low energy) bands and transmit unique hues of green and blue. The structures of

111

the polymers SprayDOTTM-Red 252, SprayDOTTM-Green 179, SprayDOTTM-Blue 153 and their

photographs are shown in Figure 5-2.

Figure 5-1 The schematic of the working principles of the 5-Electrode ECD

Figure 5-2 Chemical structures of the polymers and the photographs of their neutral (N) and doped (D) states

5.1.1 Film Deposition

In order to obtain thin films of the EC polymers for electrochemical and optical studies,

SprayDOTTM-Red 252, SprayDOTTM-Green 179 and SprayDOTTM-Blue 153 were drop-cast (on

All films neutral

All films oxidized

N O

Ox

Red

N O

Ox

Red

N O

Ox

Red

SprayDOT-Red 252 SprayDOT-Green 179 SprayDOT-Blue 153

112

a Pt-button electrode) or spray-cast (on ITO coated glass electrode) from a 2 mg/mL

polymer/toluene solution after being filtered through 0.45 μm PTFE filters. Spray cast films on

ITO/glass were dried under vacuum overnight. Non-color changing, electrochemically active

PTMA/PMMA was sprayed and treated on ITO/glass for use in absorptive/transmissive window

type ECDs, and on PETE/PEDOT:PSS/Au electrodes for use in RBG 5-ECDs as detailed in

Chapter 4.

5.1.2 Polymer CV, Scan Rate Dependence

CVs were recorded at scan rates ranging from 25 to 300 mV/s as shown in Figures 5-3 to

5-5. A linear increase of the current with scan rate is observed for each film, indicative of a

surface adhered electroactive polymer film. These polymers completed their redox cycles in

narrow potential windows. SprayDOTTM-Blue 153 was shown to switch between its extreme

redox states with a potential difference of only 0.5V. This behavior is promising for low

potential electronic device applications.

Figure 5-3 Cyclic voltammograms of SprayDOTTM- Red 252 in 0.1 M TBAP/PC at scan rates of (a) 25, (b) 50, (c) 75, (d) 100, (e) 150, (f) 200, and (g) 300 mV/s, on a Pt-button electrode. Film was prepared by drop-casting onto a Pt-button electrode from 2 mg/mL polymer/toluene solution.

-0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3-0.12-0.09-0.06-0.030.000.030.060.090.120.150.18

g

a

Cur

rent

Den

sity

(mA

/cm

2 )

Potential (V vs. Fc/Fc+)

a

g

113

Figure 5-4 Cyclic voltammograms of SprayDOTTM-Green 179 in 0.1 M TBAP/PC at scan rates of (a) 25, (b) 50, (c) 75, (d) 100, (e) 150, (f) 200, and (g) 300 mV/s, on a Pt-button electrode. Film was prepared by drop-casting onto a Pt-button electrode from 2 mg/mL polymer/toluene solution.

Figure 5-5 Cyclic voltammograms of SprayDOTTM-Blue 153 in 0.1 M TBAP/PC at scan rates of (a) 25, (b) 50, (c) 75, (d) 100, (e) 150, (f) 200, and (g) 300 mV/s, on a Pt-button electrode. Film prepared by drop-casting onto a Pt-button electrode from 2 mg/mL polymer/toluene solution.

-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4-1.2

-0.9

-0.6

-0.3

0.0

0.3

0.6

0.9

1.2

Cur

rent

Den

sity

(mA

/cm

2 )

Potential (V vs. Fc/Fc+)

a

g

a

g

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8

-3

-2

-1

0

1

2

3

Cur

rent

Den

sity

(mA

/cm

2 )

Potential (V vs. Fc/Fc+)

a

g

a

g

114

5.1.3 Spectroelectrochemistry

Spectroelectrochemical and colorimetric studies were conducted to acquire the optical

characteristics of the polymer films in the correct potential ranges determined by the CV

experiments. The spectroelectrochemical series for the polymer films are shown in Figures 5-6

to 5-8. In their neutral states, SprayDOTTM-Red 252 appears red (λmax at 528 nm), SprayDOTTM-

Green 179 appears green (λmax at 443nm and 634 nm) and SprayDOTTM-Blue 153 appears blue

(λmax at 398 nm and 652 nm). As these polymer films are doped, charge carrier states emerge

with the majority of the light absorption for each polymer being in the NIR, which results in

highly transmissive films. In case of SprayDOTTM-Green 179 and SprayDOT Blue 153 the

addition of a strong acceptor, BTD in the strong donor alkoxy thiophene based structures, red

shifts the main band while introducing a blue shifted secondary band. As seen in Figure 5-7 and

5-8 the higher energy absorption of SprayDOT-Blue 153 is more blue shifted than the

SprayDOT-Green 179. The lower contrast at the lower energy due to the enhanced near-IR

tailing results in a light blue hue in the transmissive state appearances of both green and blue

polymer films.(Figures 5-7 and 5-8)

Figure 5-6 Spectroelectrochemistry of spray-cast, redox switched, SprayDOTTM-Red 252 film on ITO/glass at applied potentials of (a) -0.47 to (o) +0.23 V vs. Fc/Fc+ at intervals of 50 mV/s in 0.1 M TBAP/PC solution.

400 600 800 1000 1200 1400 16000.0

0.2

0.4

0.6

0.8

1.0

1.2

o

a

Abs

orba

nce

(a.u

.)

wavelength (nm)

528 nm

a

o

115

Figure 5-7 Spectroelectrochemistry of spray-cast, redox switched, SprayDOTTM-Green 179 film on ITO/glass at applied potentials of (a) -0.42 to (q) +0.38 V vs. Fc/Fc+ at intervals of 50 mV/s in 0.1 M TBAP/PC solution.

Figure 5-8 Spectroelectrochemistry of spray-cast, redox switched, SprayDOTTM-Blue 153 film on ITO/glass at applied potentials of (a) +0.11 to (l) +0.66 V vs. Fc/Fc+ at intervals of 50 mV/s in 0.1 M TBAP/PC solution.

400 600 800 1000 1200 1400 16000.0

0.2

0.4

0.6

0.8

1.0

1.2

a

a

q

634 nm

Abs

orba

nce

(a.u

.)

wavelength (nm)

443 nm

q

400 600 800 1000 1200 1400 16000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

a

652 nm

Abs

orba

nce

(a.u

.)

wavelength (nm)

398 nm

a

l

l

116

5.1.4 Tandem Chronocoulometry and Chronoabsorptometry

A tandem chronoabsorptometry/chronocoulometry experiment was used to calculate

composite coloration efficiency (CE) and switch times (t0.95) at 95% of the total optical change

(%T) at λmax. The SprayDOTTM-Red 252 film with an absorbance of 0.9 a.u. at its λmax (528 nm)

has a charge density of 1.3 mC/cm2 with a 52% Δ%T of resulting in a CE of 545 cm2/C. (Figure

5-9) SprayDOTTM-Green 179 film with an absorbance of 1 a.u. at 634 nm has a charge density

of 2.1 mC/cm2 with a 50% Δ%T at 443 nm resulting in a CE of 287 cm2/C (Figure 5-10) and a

41% Δ%T at 634 nm resulting in a CE of 310 cm2/C (Figure 5-11). The SprayDOTTM-Blue 153

film with an absorbance of 1.1 a.u. at its λmax (652 nm) has a charge density of 1.7 mC/cm2 with

a 43% Δ%T resulting in a CE of 442 cm2/C. (Figure 5-12)

Switching times, which are dependent on the migration of the counterions through the

films during redox switching, were also found to be twice as much for SprayDOTTM-Red 252

when compared to SprayDOTTM-Green 179 and SprayDOT-Blue 153. All three polymers

utilized in this study have solubilizing alkoxy groups along the chains. These groups tend to

interact/entangle and close the pathway of ions. The addition of BTD to the structure in the case

of SprayDOT-Green 179 and SprayDOT-Blue 153 hinders the side chain interactions to a certain

degree and opens the morphology to allow the migration of ions in and out of the polymer matrix

resulting in faster switch times compared to SprayDOTTM-Red 252. SprayDOTTM-Red 252 has a

long switch time of 4.8 s, while SprayDOTTM-Green 179 and SprayDOTTM-Blue 153 have

switch times of 2-2.5 s and 2.2 s, respectively. SprayDOTTM-Green 179 and SprayDOT-Blue

153 also have lower optical contrast due to the high degree tailing of the NIR absorption in to the

red absorbing region in the oxidized state, thus they end up with lower CEs.

117

Figure 5-9 Tandem chronoabsorptometry and chronocoulometry experiment for SprayDOTTM-Red 252 (stepped from -0.43 V to +0.22 V vs. Fc/Fc+, held for 10 s at each potential at 528 nm) in 0.1 M TBAP/PC solution.

Figure 5-10 Tandem chronoabsorptometry and chronocoulometry experiment for SprayDOTTM-Green 179 (stepped from -0.33 V to +0.37 V vs. Fc/Fc+, held for 10 s at each potential at 443 nm) in 0.1 M TBAP/PC solution.

0 2 4 6 8 10 12 14 16 18 201.41.61.82.02.22.42.62.83.03.23.4

time (sec.)

Cha

rge

Den

sity

(mC

/cm

2 )

10

20

30

40

50

60

70

% Transm

ittance

0 2 4 6 8 10 12 14 16 18 20

1.0

1.5

2.0

2.5

3.0

3.5

time (sec.)

Cha

rge

Den

sity

(mC

/cm

2 )

10

20

30

40

50

60

70

% Transm

ittance

118

Figure 5-11 Tandem chronoabsorptometry and chronocoulometry experiment for SprayDOTTM-Green 179 (stepped from -0.33 V to +0.37 V vs. Fc/Fc+, held for 10 s at each potential at 634 nm) in 0.1 M TBAP/PC solution.

Figure 5-12 Tandem chronoabsorptometry and chronocoulometry experiment for SprayDOTTM-Blue 153 (stepped from -0.13 V to +0.57 V vs. Fc/Fc+, held for 10 s at each potential at 652 nm) in 0.1 M TBAP/PC solution.

0 2 4 6 8 10 12 14 16 18 20

0.5

1.0

1.5

2.0

2.5

3.0

3.5

time (sec.)

Cha

rge

Den

sity

(mC

/cm

2 )

10

20

30

40

50

60

% Transm

ittance

0 2 4 6 8 10 12 14 16 18 201.5

2.0

2.5

3.0

3.5

4.0

time (sec.)

Cha

rge

Den

sity

(mC

/cm

2 )

10

20

30

40

50

60

% Transm

ittance

119

5.1.5 Colorimetry

In situ color coordinates and relative luminance values were recorded for each of the three

electrochromic polymer films with primary colors in their fully reduced states. (Figures 5-13 to

5-15) SprayDOTTM-Red 252 has a* and b* values of 49 and 5, respectively, giving it a red color

with a relative luminance of 28%, SprayDOTTM-Green 179 has a* and b* values of -15 and -5,

respectively, giving it a green color with a relative luminance of 25%, and SprayDOTTM-Blue

153 has a* and b* values of -21 and -36 giving it a blue color with a relative luminance of 25%.

When the films are completely oxidized they all are converted into highly transmissive

states. Now, SprayDOTTM-Red 252 exhibits a* and b* values of -1 and 2, respectively, with a

relative luminance of 69%, SprayDOTTM-Green 179 a* and b* values of -3 and -8, respectively,

with a relative luminance of 64%, and SprayDOTTM-Blue 153 exhibits a* and b* values of -4

and -2, respectively, with a relative luminance of 65%.

All three cathodically coloring polymers possess a luminance contrast ratio of C ~0.4

having Δ%Y of ~40% from fully neutral to the oxidized form. It is wise to mention that as

shown in Chapter 3, PEDOT has luminance contrast ratio of 0.4 with a Δ%Y of 50%. As a result

of optical studies, films with absorbance values of 1 a.u. at their λmax were utilized in the device

applications. Increasing the thickness of the films above that value results in a lower contrast

ratio and decreasing it results in less saturated colors. The limited contrast of these films

hindered the contrast of the ECDs.

As shown in Figures 5-13 to 5-15, the change of relative luminance as a function of

applied potential show hysteresis. In other words, the optical responses of EC polymer films are

path dependent, and the films behave differently upon oxidation and reduction. When the EC

120

polymer film is first oxidized an electron is removed from the top of valence band and a radical

cation with a partial quinoid structure forms and intermediate polaronic levels emerge. The

reduction of the film in the reverse direction results in a diradical due to the addition of electron

to the newly formed, half-filled intermediate energy levels (polaronic states). After that addition,

the polymer chain spontaneously relaxes to its neutral state. The difference in the mechanism

and energy results in lower or higher potentials for optical changes upon reduction and oxidation,

respectively.17

Figure 5-13 Relative luminance as a function of applied potential and L*a*b* color coordinates and photography at redox extremes of SprayDOTTM-Red 252 in 0.1 M TBAP/PC

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.620

30

40

50

60

70

Rel

ativ

e Lu

min

ance

(% Y

)

Potential (V vs. Fc/Fc+)

L*=60a*=49b*= 5

L*=87a*=-1b*= 2

121

Figure 5-14 Relative luminance as a function of applied potential and L*a*b* color coordinates and photography at redox extremes of SprayDOTTM-Green 179 in 0.1 M TBAP/PC.

Figure 5-15 Relative luminance as a function of applied potential and L*a*b* color coordinates and photography at redox extremes of SprayDOTTM-Blue 153 in 0.1 M TBAP/PC.

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.620

30

40

50

60

70

Rel

ativ

e Lu

min

ance

(% Y

)

Potential (V vs. Fc/Fc+)

L*= 57a*=-15b*= -5

L*=84a*= -3b*= -8

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.620

30

40

50

60

70

Rel

ativ

e Lu

min

ance

(% Y

)

Potential (V vs. Fc/Fc+)

L*= 57a*=-21b*=-36

L*= 84a*= -4b*= -2

122

5.2 Dual Absorptive/Transmissive Window ECDs

After the completion of the optical characterizations, the behaviors of the polymers were

studied in standard dual absorptive/transmissve window type ECDs. The schematic and the

construction details of the devices are given in Chapters 1 and 2. EC polymers were sprayed

onto ITO/glass electrodes and dried under vacuum. A layer of PTMA spray on ITO/glass was

followed by a spray of PTMA/PMMA solution until the film becomes opaque white. Once

placed in the electrolyte, the film becomes transmissive. Film thicknesses were set to balance

the number of redox sites on the cathodically coloring and noncolor changing film couples. All

the electroactive films were conditioned by potential cycling. Cathodically coloring red, green

and blue polymer films were doped (bleached) and PTMA/PMMA films were reduced prior to

the device assembly to start with an initial charge balance. Cathodically coloring films were

coated with a TBAP/PC gel electrolyte and sandwiched with the counter electrodes,

ITO/PTMA/PMMA. Devices were encapsulated by paraffin wax and the results obtained from

these separate applications of the films let us set the baseline properties and foresee their

behavior in more complicated systems. Spectroelectrochemical and colorimetric studies were

conducted to acquire the optical characteristics of the window devices.

The spectroelectrochemical data for these Red,. Green and Blue ECDs are shown in

Figures 5-16 to 5-18 with the photographs of the associated colors. When the devices were

negatively biased to their colored states, SprayDOTTM-Red 252/PTMA device appears red

(absorbing mainly at 532 nm) and has L*, a*, b* coordinates of 58,38,6, respectively,

SprayDOTTM-Green 179/PTMA device appears green (absorbing at 443nm and 643 nm) and has

L*, a*, b* coordinates of 45,-13,-1, respectively, and SprayDOTTM-Blue 153/PTMA device

appears blue (absorbing at 398 nm and 652 nm) and has L*, a*, b* coordinates of 55,-18,-35,

respectively. When the bias was reversed, devices switched to their transmissive states,

123

SprayDOTTM-Red 252/PTMA device maintains a yellow hue and has L*, a*, b* coordinates of

81,-2,3, respectively, SprayDOTTM-Green 179/PTMA device maintains a blue hue and has L*,

a*, b* coordinates of 69,-4,-10, respectively, and SprayDOTTM-Blue 153/PTMA device

maintains a blue hue and has L*, a*, b* coordinates of 80, -3, -5, respectively. Even though the

dual ECDs showed good contrast and saturated colors their limited stabilities hindered the

efficiency of the devices.

Figure 5-16 SprayDOTTM-Red 252/PTMA Window ECD. UV-vis-NIR spectra and the L*a*b* color coordinates with the associated photographs at the redox extremes.

Figure 5-17 SprayDOTTM-Green 179 /PTMA Window ECD. UV-vis-NIR spectra and the L*a*b* color coordinates with the associated photographs at the redox extremes.

400 600 800 1000 1200 1400 16000.0

0.2

0.4

0.6

0.8

1.0

Abs

orba

nce

(a.u

.)

wavelength (nm)

L*= 58a*= 38b*= 6

L*= 81a*= -2b*= 3

± 1.5 V

400 600 800 1000 1200 1400 1600

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Abs

orba

nce

(a.u

.)

wavelength (nm)

L*= 45a*= -13b*= -1

L*= 69a*= -4b*=-10

± 2 V

124

Figure 5-18 SprayDOTTM-Blue 153 /PTMA Window ECD. UV-vis-NIR spectra and the L*a*b* color coordinates with the associated photographs at the redox extremes.

5.3 RGB Color Space 5-Electrode ECD

The RGB 5-ECD reported here consists of three working electrodes and two counter

electrodes whose potentials are controlled separately as demonstrated by the schematic diagram

in Figure 5-19. ITO/glass was chosen as the electrode material for the outermost electrodes,

working electrode 1 and working electrode 3, in order to establish a strong foundation for the

multi-layered device. Inner electrodes were constructed using porous PETE/PEDOT:PSS/Au as

detailed in Chapter 4. Electrochromic polymers which are red (SprayDOTTM-Red 252) and

green (SprayDOTTM-Green 179) at their neutral states were sprayed on to ITO/glass electrodes

and named working electrodes 1 and 3, respectively. Electrochromic polymer which is blue

(SprayDOTTM-Blue 153) at its neutral state was sprayed on PETE/PEDOT:PSS/Au electrode and

named working electrode 2. PTMA/PMMA the non-electrochromic, electroactive, transparent

polymer blend, was sprayed on the porous PETE/PEDOT:PSS/Au electrode and treated as

detailed in Chapter 4 to be utilized as counter electrodes 1 and 2. Counter electrodes 1 and 2

were sandwiched in between the working electrodes 1, 2 and 3 and maintained the charge

400 600 800 1000 1200 1400 16000.00.20.40.60.81.01.21.41.61.82.0

Abs

orba

nce

(a.u

.)

wavelength (nm)

L*= 55a*= -18b*= -35

L*= 80a*= -3b*= -5

± 2 V

125

balance in the device. The electrode layers were separated by the TBAP/PC gel electrolyte,

which utilizes the charge transport. The device was encapsulated with a paraffin wax. The

potential control of the 5 electrode device was facilitated by a potentioastat and a bipotentiostat.

Spectroelectrochemical and colorimetric data were obtained from the device (Figures 5-20

to 5-21). When the green and blue films were doped to their transmissive states (at +2.5 V each)

and the red film was neutralized at its colored state (at -2.5 V) the device appears red absorbing

mainly at 532 nm (Figure 5-20 (a)). When the red and blue films were doped to their

transmissive states and the green film was neutralized at its colored state the device appears

green absorbing mainly at two bands of 434 and 647 nm (Figure 5-20 (b)). When the red and

green films were doped to their transmissive states and the blue film was neutralized at its

colored state the device appears blue absorbing at two bands of 398 and 634 nm (Figure 5-20

(c)). The photographs with the associated color coordinates and the chromaticity diagram of the

thrichromatic states are shown in Figure 5-22. The blue triangle represents the color gamut of

RGB 5-ECD and the black triangle represents the color gamut for CRT display. The

chromaticity coordinates far from the spectral locus on chromaticity diagram indicates the low

purity of the colors we achieved from the device. The device appeared red with L*, a* and b*

values of 32, 20 and 1, respectively, appeared green with L*, a* and b* values of 42, -6 and -1,

respectively, and appeared blue with L*, a* and b* values of 37, -11 and -23, respectively. Pure

and more saturated colors could be achieved by further optimization of the counter electrode

which adds a blue hue to the overall device because of the reflectance of PEDOT:PSS and Au.

In addition to that, polymer structures could be tailored further towards obtaining more saturated

colors. The color purity (saturation) decreases from a single EC film to dual ECD and to RGB 5-

126

ECD as shown by the the changes in a* and b* values in Table 5-1. It is promising since the

change from single layer film to multi-electrode devices is not drastic.

Figure 5-19 Schematic of the RGB 5- ECD

Figure 5-20 UV–vis-NIR spectra of the individual colors obtained from the RGB 5-ECD, (a) red, (b) green and (c) blue.

WE 1

WE 3

CE 1

CE 2

WE 1 WE 3WE 2

CE 1 CE 2

Pot.

Bipot.

CERE

WE2WE3

WE1CE

RE

400 600 800 1000 1200 1400 1600

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

a

b

c

c

b

Abs

orba

nce

(a.u

.)

wavelength (nm)

a

127

Figure 5-21 L*a*b* color coordinates and photography of the RGB 5-ECD. CIE chromaticity diagram with chromaticity coordinates shown for the RGB 5-ECD (blue triangle) and the CRT phosphors (black triangle).

Table 5-1 Change in a*/b* values from a single EC film to multi-electrode devices a*/b* Red Green Blue Film 49/5 -15/-5 -21/-36 Dual ECD 38/6 -13/-1 -18/-35 RGB 5-ECD 20/1 -6/-1 -11/-23

5.4 Conclusions and Future Perspectives

Entire range of colors to be used in display applications is accessible if we have three

primary colors. In this project, three primaries, SprayDOTTM-Red 252, SprayDOTTM-Green 179

and SprayDOTTM-Blue 153, were utilized in the RGB 5-ECD. The display device consisted of

five electrodes of which three have red, green and blue electrochromic polymers sprayed on

them. Highly conductive, transmissive and porous counter electrode, PETE/PEDOT:PSS/Au

and noncolor changing, electroactive PTMA was used as a counter electrode material to supply

charge and prevent early degradation. In this device red, green and blue electrodes are stacked

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

..

.y

x

.

700 nm

400 nm

.

.

L*= 32a*= 20b*= 1

L*= 42a*= -6b*= -1

L*= 37a*= -11b*= -23

128

on top of each other. The separate control of three primary colors allowed the generation of a

RGB color spaces through one single pixel.

Today display technology uses red, green and blue phosphors on adjacent pixels. When

viewed from a proper distance the mixing of all these regular sequence of red, green and blue

stripes or rectangles gives an impression of white. By varying the intensity of light at each pixel

different colors can be achieved. In the second part of this project conductive, transparent

electrodes will be patterned to regular sequences of dots, rectangles or stripes and they will

sprayed with red, green and blue polymers. Separate control of three primary colors and their

intensities will generate full RGB color space. The methodologies used by our group in the past

can be revisited to utilize patterning and printing techniques into RGB 5-ECD application. In

order to access all the colors along the visible spectrum, Red, Orange, Yellow, Green, Blue,

Indigo and Violet, different primaries must be utilized. These primaries are red, blue and yellow.

Having red and blue to transmissive switching EC polymers available, this goal could be

achieved by sysnthesis of yellow to transmissive switching EC polymer.

129

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134

BIOGRAPHICAL SKETCH

Ece Unur was born in 1979 in Bursa, Turkey, to Havva and Irfan Unur. She has a younger

brother, Lutfu and an older sister, Necibe. She started school in Bursa, and then moved to a

boarding school in Istanbul. She stayed there for seven years. She started college in Ankara at

Middle East Technical University in 1998. She got her B.S. in chemistry in 2002 and the next

year she received her M.Sc. in chemistry and her double major B.S. in chemical engineering. In

August 2003 she moved to Gainesville, Florida to pursue a Ph.D. degree and joined the

Reynolds’ Group.